FORM OF LETTER OF TRANSMITTAL

To Tender Preferred Shares (Designated Municipal Auction Rate Cumulative Preferred Shares Series F)

THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 8:00 A.M., NEW YORK CITY TIME, ON SEPTEMBER 7, 2012, UNLESS THE OFFER IS EXTENDED.

Delivery by First Class Mail, By Registered, Certified or Express Mail, By Overnight Courier, or By Hand should be directed to:

DELIVERY OF THIS LETTER OF TRANSMITTAL TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE WILL NOT CONSTITUTE A VALID DELIVERY.

THE INSTRUCTIONS ACCOMPANYING THIS LETTER OF TRANSMITTAL SHOULD BE READ CAREFULLY BEFORE THIS LETTER OF TRANSMITTAL IS COMPLETED.

This Letter of Transmittal relates to the offer by MFS High Yield Municipal Trust (the “Fund”), a Massachusetts business trust registered under the Investment Company Act of 1940, as amended, to purchase for cash up to 100% of its outstanding preferred shares of beneficial interest, without par value and liquidation preference of $25,000 per share, designated Municipal Auction Rate Cumulative Preferred Shares Series F (the “Preferred Shares”) upon the terms and subject to the conditions set forth in the offer to purchase dated August 9, 2012 (the “Offer to Purchase”), receipt of which is hereby acknowledged, and in this Letter of Transmittal, which, together with the Offer to Purchase, each as amended or supplemented from time to time, together constitute the “Offer.”

If you wish to tender all or any part of your Series F Preferred Shares of the Funds, you should either:

If you desire to tender Series F Preferred Shares and such Series F Preferred Shares cannot be delivered to the Depositary or you cannot comply with the procedure for book-entry transfer or your other required documents cannot be delivered to the Depositary, in any case, by the expiration of the Offer, you must tender such Series F Preferred Shares pursuant to the guaranteed delivery procedure set forth in Section 3 of the Offer to Purchase.

Questions and requests for assistance or for additional copies of this Letter of Transmittal, the Offer to Purchase or the Notice of Guaranteed Delivery or the Notice of Withdrawal may be directed to Deutsche Bank Trust Company Americas (“Depositary”) at its address or telephone number set forth on the first page of this Letter of Transmittal.

The undersigned hereby tenders to MFS High Yield Municipal Trust, a Massachusetts business trust (the “Fund”), the above-described preferred shares of beneficial interest, without par value per share and liquidation preference of $25,000 per share, designated Municipal Auction Rate Cumulative Preferred Shares Series F, pursuant to the Fund’s offer to purchase for cash up to 100% of its outstanding Preferred Shares, upon the terms and subject to the conditions set forth in the Fund’s Offer to Purchase dated August 9, 2012 (the “Offer to Purchase”), receipt of which is hereby acknowledged, and this Letter of Transmittal which, together with the Offer to Purchase, each as amended or supplemented from time to time, together constitute the “Offer.” The price to be paid for the Preferred Shares is an amount per share, net to the seller in cash, equal to 95% of the liquidation preference per share (or $23,750 per share), plus any unpaid dividends accrued through September 7, 2012, or such later date to which the Offer is extended. The Offer by the Fund is conditioned upon there being validly tendered and not withdrawn prior to the expiration of the Fund’s offer that number of Preferred Shares (as defined herein) which represent at least 70% of the Fund’s outstanding Preferred Shares, the Fund’s issuance of privately placed new preferred shares (“New Preferred Shares”) and the approval of the proposals to amend the Fund’s Bylaws to replace Standard & Poor’s, a division of The McGraw-Hill Companies, Inc. with Fitch, Inc. as a rating agency for the Preferred Shares as set forth in the proxy statement dated August 9, 2012.

Upon the terms and subject to the conditions of the Offer and effective upon acceptance for payment of and payment for the Preferred Shares tendered herewith, the undersigned hereby sells, assigns and transfers to, or upon the order of, the Fund all right, title and interest in and to all the Preferred Shares that is being tendered hereby and appoints the Depositary as the true and lawful agent and attorney-in-fact of the undersigned with respect to such Preferred Shares, with full knowledge that the depositary also acts as the agent of the Fund, with full power of substitution (such power of attorney being deemed to be an irrevocable power coupled with an interest), to:

All authority herein conferred or agreed to be conferred by this Letter of Transmittal shall survive the death or incapacity of the undersigned, and any obligation of the undersigned hereunder shall be binding upon the heirs, personal representatives, executors, administrators, successors, assigns, trustees in bankruptcy and legal representatives of the undersigned. Except as stated in the Offer to Purchase, this tender is irrevocable.

The purchase price of each Preferred Share will equal 95% of the liquidation preference per share (or $23,750 per share), plus any unpaid dividends accrued through September 7, 2012, or such later date to which the Offer is extended. All Preferred Shares validly tendered on or before the expiration date of the Offer and not properly withdrawn will be purchased, subject to the terms and conditions of the Offer. If any tendered Preferred Shares are not accepted for payment pursuant to the terms and conditions of the Offer for any reason, such Preferred Shares will be returned without expense to the holder of Preferred Shares (“Preferred Shareholder”) in accordance with Section 5 of the Offer to Purchase.

The undersigned understands that tenders of Preferred Shares pursuant to any one of the procedures described in Section 3 of the Offer to Purchase and in the instructions hereto will constitute a binding agreement between the undersigned and the Fund upon the terms and subject to the conditions of the Offer. The undersigned acknowledges that under no circumstances will the Fund pay interest on the purchase price, including, without limitation, by reason of any delay in making payment.

The undersigned recognizes that under the circumstances set forth in the Offer to Purchase, the Fund may terminate or amend the Offer; may postpone the acceptance for payment of, or the payment for, Preferred Shares tendered; or may accept for payment fewer than all of the Preferred Shares tendered.

A wire for the aggregate net purchase price for the Preferred Shares tendered and purchased will sent to the account at the Book-Entry Transfer Facility.

SIGN HERE

(Please complete and return the Substitute Form W-9 below)*

(Signature(s) of Owners)



Dated:		__________________

Name(s):

(Please Print)

Capacity (full title and location signed): ________________________________________


Address:

(Include Zip Code)

Area Code and Telephone Number: ____________________________________________

(Must be signed by registered holder(s) exactly as name(s) appear(s) on a security position listing or by person(s) authorized to become registered holder(s) by certificate(s) and documents transmitted herewith. If signature is by a trustee, executor, administrator, guardian, attorney-in-fact, agent, officer of a corporation or other person acting in a fiduciary or representative capacity, please set forth full title and location of signing and see Instruction 4.)

Guarantee Of Signature(s)

(See Instructions 1 and 4)

Authorized Signature:

Name:

Title:

(Please Type or Print)

Name of Firm:

Address:

(Include Zip Code)

Area Code and Telephone No.:

Dated:

SUBSTITUTE

FORM W-9

Department of the

Treasury/Internal

Revenue Service

Payer’s Request for Taxpayer Identification

Number and Certification

Part I

Taxpayer Identification No. — For All Accounts

Enter your taxpayer identification number in the appropriate box.

For most individuals and sole proprietors, this is your Social Security Number. For other entities, it is your Employer Identification Number. If you do not have a number, see “How to Obtain a TIN” in the “Guidelines For Certification of Taxpayer

Identification Number on Substitute Form W-9” (“Guidelines”) below. Note: If the account is in more than one name, see the chart on page 1 of the Guidelines to determine what number to enter.

Social Security Number

_____________________

Employee Identification Number

_____________________

Part II

For Payees Exempt From Backup Payer’s Request for Withholding (see Guidelines)

Certification – Under penalties of perjury, I certify that:

(1)

The number shown on this form (i) is my correct Taxpayer Identification Number or (ii) I am waiting for a number to be issued to me and either (a) I have mailed or delivered an application to receive a taxpayer identification number to the appropriate Internal Revenue Service Center or Social Security Administration Office or (b) I intend to mail or deliver an application in the near future. I understand that if I do not provide a taxpayer identification number within sixty (60) days, all reportable payments made to me thereafter will be subject to backup withholding until I provide a number, and certain reportable payments may be subject to withholding within that sixty (60) day period until I have provided a taxpayer identification number;

(2)

I am not subject to backup withholding either because (a) I am exempt from backup withholding, (b) I have not been notified by the Internal Revenue Service (“IRS”) that I am subject to backup withholding as a result of a failure to report all interest or dividends, or (c) the IRS has notified me that I am no longer subject to backup withholding;

(3)

I am a U.S. citizen or other U.S. person (including a U.S. resident alien (as defined in the enclosed Guidelines)); and

(4)

Any information provided on this Substitute Form W-9 is true, correct and complete.

(Please cross out certification (2) if you have been notified by the IRS that you are currently subject to backup withholding because you have failed to report all interest and dividends on your tax return.)


Signature of U.S. Person:				Date:


Name (as shown on your income tax return):


Business name (if different from above):


Address:


City: ____________________________		State: ___________________________		Zip: _____________________

Check the appropriate box:


¨ Individual/Sole Proprietor	¨ Corporation	¨ S Corporation
¨ Partnership	¨ Trust/Estate	¨ Other ____________
¨ Limited Liability Company. Enter the tax classification (S = S corporation,
C = corporation, P = partnership): ________________
¨ Exempt from backup withholding

1. Guarantee of Signatures. Except as otherwise provided below, all signatures on this Letter of Transmittal must be guaranteed by a financial institution (including most banks, savings and loan associations and brokerage houses) that is a member of a recognized Medallion Program approved by The Securities Transfer Association, Inc., including the Securities Transfer Agents Medallion Program (STAMP), or any other “eligible guarantor institution” (as such term is defined in Rule 17Ad-15 under the Securities Exchange Act of 1934, as amended) (each an “Eligible Institution”). Signatures on this Letter of Transmittal need not be guaranteed if such Preferred Shares are tendered for the account of an Eligible Institution. See Instruction 4.

2. Delivery of Preferred Shares. Delivery of Preferred Shares is to be made by book-entry delivery pursuant to the procedures set forth in Section 3 of the Offer to Purchase; an Agent’s Message (as defined in the Offer to Purchase) should be utilized. A confirmation of a book-entry transfer into the Depositary’s account at the Book-Entry Transfer Facility of all Preferred Shares delivered electronically, as well as an Agent’s Message, and any other documents required by this Letter of Transmittal, must be received by the Depositary at its address set forth on the front page of this Letter of Transmittal by the Expiration Date (as defined in the Offer to Purchase). Preferred Shareholders who cannot deliver their Preferred Shares and all other required documents to the Depositary by the Expiration Date must tender their Preferred Shares pursuant to the guaranteed delivery procedure set forth in Section 3 of the Offer to Purchase. Pursuant to such procedure: (i) such tender must be made by or through an Eligible Institution, (ii) a properly completed and duly executed Notice of Guaranteed Delivery substantially in the form provided by the Fund must be received by the Depositary by the Expiration Date and (iii) a properly completed and duly executed Letter of Transmittal (or facsimile thereof) with any required signature guarantee or an Agent’s Message and any other documents required by the Letter of Transmittal and, for Preferred Shares held in street name, confirmation of a book-entry transfer of such Preferred Shares into the Depositary’s account at the Book-Entry Transfer Facility, must be received by the Depositary within three NYSE trading days after the date of execution of the Notice of Guaranteed Delivery, all as provided in Section 3 of the Offer to Purchase.

The method of delivery of any documents is at the option and risk of the tendering Preferred Shareholder. If delivery is by mail, registered mail with return receipt requested, properly insured, is recommended. In all cases, sufficient time should be allowed to ensure timely delivery.

No alternative, conditional or contingent tenders will be accepted, and no fractional Preferred Shares will be purchased. By executing this Letter of Transmittal, the tendering Preferred Shareholder waives any right to receive any notice of the acceptance for payment of the Preferred Shares.

3. Partial Tenders. If any tendered Preferred Shares are not accepted for payment pursuant to the terms and conditions of the Offer for any reason, or if any tendered Preferred Shares are not accepted because of an invalid tender, or if any tendered Preferred Shares are properly withdrawn, Preferred Shares will be returned to the appropriate account at the Book-Entry Transfer Facility without charge by the Fund to the tendering Preferred Shareholder, as soon as practicable following expiration or termination of the Offer or the proper withdrawal of the Preferred Shares.

4. Stock Transfer Taxes. The Fund will pay any stock transfer taxes with respect to the sale and transfer of any Preferred Shares to it or its order pursuant to the Offer. If, however, a transfer tax is imposed for any reason other than the sale or transfer of Preferred Shares to the Fund pursuant to the Offer, then the amount of any stock transfer taxes (whether imposed on the registered holder(s), such other person or otherwise) will be deducted from the purchase price unless satisfactory evidence of the payment of such taxes, or exemption therefrom, is submitted herewith.

5. Substitute Form W-9. Under U.S. federal income tax law, a backup withholding tax of 28% may be imposed on the gross proceeds otherwise payable to certain Preferred Shareholders pursuant to the Offer. In order to avoid such backup withholding, each tendering Preferred Shareholder must provide the applicable

withholding agent with such Preferred Shareholder’s correct taxpayer identification number and certify that such Preferred Shareholder or payee is not subject to such backup withholding by completing the Substitute Form W-9 set forth above (or, in the case of a foreign Preferred Shareholder, an appropriate IRS Form W-8 or Substitute Form W-8). In general, if a Preferred Shareholder is an individual, the taxpayer identification number is the Social Security number of such individual. If the applicable withholding agent is not provided with the correct taxpayer identification number, the Preferred Shareholder will be subject to a $50 penalty imposed by the Internal Revenue Service unless the failure to provide the correct number is due to reasonable cause and not willful neglect. A false statement made on the Substitute Form W-9 without any reasonable basis that results in no backup withholding will be subject to a $500 penalty, and the willful falsification of certifications or affirmations may be subject to criminal penalties, including fines and/or imprisonment. Certain Preferred Shareholders (including, among others, all corporations and certain foreign individuals) are not subject to these backup withholding and reporting requirements. However, such Preferred Shareholders should complete the Substitute Form W-9 (or, in the case of a foreign Preferred Shareholders, an appropriate IRS Form W-8 or Substitute Form W-8) to avoid erroneous backup withholding. For further information concerning backup withholding and instructions for completing the Substitute Form W-9 (including how to obtain a taxpayer identification number if you do not have one and how to complete the Substitute Form W-9 if Preferred Shares is held in more than one name), consult the Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9.

Foreign persons should not complete a Substitute Form W-9. Instead, in order to establish that a foreign person qualifies as an exempt recipient for which backup withholding is not required, such person should submit an appropriate IRS Form W-8 or Substitute Form W-8, attesting to its status. These forms can be obtained from the Depositary or from the relevant broker, dealer, commercial bank, trust company or other nominee.

Backup withholding is not an additional federal income tax. Rather, the federal income tax liability of a person subject to backup withholding will be reduced by the amount of tax withheld. If withholding results in an overpayment of taxes, a refund may be obtained provided that the required information is furnished to the Internal Revenue Service. NOTE: FAILURE TO COMPLETE AND RETURN THE SUBSTITUTE FORM W-9 (OR AN APPROPRIATE IRS FORM W-8 OR SUBSTITUTE FORM W-8) MAY RESULT IN PENALTIES AND BACKUP WITHHOLDING ON ANY AMOUNTS OTHERWISE PAYABLE TO YOU PURSUANT TO THE OFFER. PLEASE REVIEW THE GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9 FOR ADDITIONAL DETAILS.

6. Waiver of Conditions. Subject to the Offer to Purchase, the conditions of the Offer may be waived, in whole or in part, by the Fund, at any time and from time to time, in the case of any Preferred Shares tendered.

7. Irregularities. The Fund will determine, in its sole discretion, all questions as to the number of Preferred Shares to be accepted, and the validity, form, eligibility, including time of receipt, and acceptance for payment of any tender of Preferred Shares. The Fund’s determination will be final and binding on all parties. The Fund reserves the absolute right to reject any or all tenders it determines not to be in proper form or the acceptance of or payment for which it determines may be unlawful. The Fund also reserves the absolute right to waive any defect or irregularity in the tender of any particular Preferred Shares or any particular Preferred Shareholder. No tender of Preferred Shares will be deemed to be properly made until all defects or irregularities have been cured by the tendering Preferred Shareholder or waived by the Fund. None of the Fund, the Depositary, the Information Agent, or any other person will be under any duty to give notice of any defects or irregularities in any tender, or incur any liability for failure to give any such notice. The Fund’s interpretation of the terms of and conditions to the Offer, including this Letter of Transmittal and the instructions thereto, will be final and binding. By tendering Preferred Shares to the Fund, you agree to accept all decisions the Fund makes concerning these matters and waive any right you might otherwise have to challenge those decisions.

8. Acceptance of Tendered Shares. If a Preferred Shareholder tenders all of his or her Preferred Shares, all such Preferred Shares credited to such Preferred Shareholder’s account(s) will be tendered unless the Preferred Shareholder otherwise specifies.

9. Contact Information. In order to facilitate the Offer and any auctions for Preferred Shares that may remain outstanding after the Offer is completed, each broker or other Nominee Holder must provide additional contact information for its Auction Department, or whoever at the broker or other Nominee Holder submits auction instructions for the Preferred Shares on its behalf, and/or the Broker-Dealer, if a different party, that submits those auction instructions to the Auction Agent. If you are unable to provide this contact information, the Fund, in its sole discretion, may waive this requirement.

Any questions and requests for assistance may be directed to the Information Agent at its address and telephone number set forth on the first page of this Letter of Transmittal. Additional copies of the Offer to Purchase, the Letter of Transmittal and Notice of Guaranteed Delivery may be obtained from the Information Agent at its address and telephone number set forth on the first page of this Letter of Transmittal. Preferred Shareholders may also contact their broker, dealer, commercial bank or trust company or other nominee for assistance concerning the Offer.

Guidelines for Determining the Proper Taxpayer Identification Number for the Payee (You) to Give the Payer.—A Social Security number and an individual taxpayer identification number each has nine digits separated by two hyphens: i.e., 000-00-0000. An employer identification number has nine digits separated by only one hyphen: i.e., 00-0000000. The table below will help determine the proper number to give the payer. All “Section” references are to the Internal Revenue Code of 1986, as amended. “IRS” is the Internal Revenue Service.

If you do not have a taxpayer identification number, obtain Form SS-5 (Application for a Social Security Card) at the local Social Security Administration (“SSA”) office or Form SS-4 (Application for Employer Identification Number) or Form W-7 (Application for IRS Individual Taxpayer Identification Number) from the SSA or IRS or by calling (800) TAX-FORM, and apply for a number.

A person that is not a U.S. person is a foreign person. If you are a foreign person, do not use this Substitute Form W-9. Instead, use the appropriate IRS Form W-8 or Substitute Form W-8, which can be obtained from the Depositary.

Payees specifically exempted from backup withholding on all reportable payments include the following:

Payees that may be specifically exempted from backup withholding include the following:

Payments of dividends and interest generally exempt from backup withholding include, but are not limited to:

Certain payments that are exempt from information reporting are also exempt from backup withholding. For details, see Sections 6041, 6041A, 6042, 6044, 6045, 6049, 6050A and 6050N.

U.S. EXEMPT PAYEES DESCRIBED ABOVE MUST FILE FORM W-9 OR A SUBSTITUTE FORM W-9 TO AVOID POSSIBLE ERRONEOUS BACKUP WITHHOLDING. FILE THAT FORM WITH THE PAYER. FURNISH YOUR TAXPAYER IDENTIFICATION NUMBER ON THE FORM, CHECK THE BOX LABELED “EXEMPT FROM BACKUP WITHHOLDING” ON THE FORM, SIGN AND DATE THE FORM AND RETURN IT TO THE PAYER.

Privacy Act Notice—Section 6109 requires most recipients of dividends and certain other payments to provide a correct taxpayer identification number to payers, who must report the payments to the IRS. The IRS uses the number for identification purposes and may also provide this information to various government agencies for tax enforcement or litigation purposes. Payers must be given the numbers whether or not recipients are required to file tax returns. Payers must generally withhold at least 28% of taxable interest, dividend, and certain other payments to a payee who does not furnish a taxpayer identification number to a payer. Certain penalties may also apply.

(1) Failure to Furnish Taxpayer Identification Number. If you fail to furnish your taxpayer identification number to a payer, you are subject to a penalty of $50 for each such failure unless your failure is due to reasonable cause and not to willful neglect.

(2) Civil Penalty for False Information with Respect to Withholding. If you make a false statement with no reasonable basis that results in no backup withholding, you are subject to a $500 penalty.

(3) Criminal Penalty for Falsifying Information. Willfully falsifying certifications or affirmations may subject you to criminal penalties including fines and/or imprisonment.

(4) Misuse of Taxpayer Identification Numbers. If the requester discloses or uses taxpayer identification numbers in violation of federal law, the requester may be subject to civil and criminal penalties.

FOR ADDITIONAL INFORMATION CONTACT YOUR TAX CONSULTANT OR THE INTERNAL REVENUE SERVICE.



For this type of account:		Give the SOCIAL SECURITY NUMBER or INDIVIDUAL TAXPAYER IDENTIFICATION NUMBER of—

1. An individual		The individual

2. Two or more individuals (joint account)		The actual owner of the account or, if combined funds, the first individual on the account⁽¹⁾

3. Custodian account of a minor (Uniform Transfers/Gift to Minors Act)		The minor⁽²⁾

4. (a) Usual revocable savings trust account (grantor is also trustee)		The grantor-trustee⁽¹⁾

(b) So-called trust account that is not a legal or valid trust under state law		The actual owner⁽¹⁾

5. Sole proprietorship or single-member limited liability company (“LLC”) that is a disregarded entity		The member⁽³⁾

6. Grantor trust filing under Optional Form 1099 Filing Method 1 (see Regulation 1.671-4(b)(2)(i)(A))		The grantor*

For this type of account:		EMPLOYER IDENTIFICATION NUMBER of—

7. Valid trust, estate, or pension trust		The legal entity⁽⁴⁾

8. Corporation or entity electing corporate status on IRS Form 8832 or Form 2553		The corporation or entity

9. Religious, charitable, or educational organization or association, club, or other tax-exempt organization		The organization

10. Partnership or multi-member LLC		The partnership or LLC

11. Broker or registered nominee		The broker or nominee

12. Account with the Department of Agriculture in the name of a public entity (such as a state or local government, school district, or prison) that receives agricultural program payments		The public entity

13. Grantor trust filing under the Form 1041 Filing Method or the Optional Form 1099 Filing Method 2 (see Regulation 1.671-4(b)(2)(i)(B))		The trust



Contact Person in Auction Department of Tendering Institution*



Broker-Dealer who submits auction instructions to the Auction Agent on your behalf


Name(s) of Tendering Holder(s) of Preferred Shares

Date of Execution of Notice of Guaranteed Delivery

Name of Institution which Guaranteed Delivery



Email Address of Contact Person in Auction Department*