UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

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Rule 13d-1(b)

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Rule 13d-1(c)

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Rule 13d-1(d)

CUSIP No.  909205106

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
White Mountains Insurance Group, Ltd. “White Mountains”

(No. 94-2708455)

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
Bermuda

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
0

6.

Shared Voting Power 
2,293,100*

7.

Sole Dispositive Power 
0

8.

Shared Dispositive Power
2,293,100*

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
2,293,100*

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)
6.7%

12.

Type of Reporting Person (See Instructions)
HC, CO

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
White Mountains Advisors, LLC. “Advisors” (No. 04-6140276)

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
2,265,000**

6.

Shared Voting Power 
0

7.

Sole Dispositive Power 
2,265,000**

8.

Shared Dispositive Power
0

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
2,265,000**

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)
6.6%

12.

Type of Reporting Person (See Instructions)
IA, CO

Item 1.

(a)

Name of Issuer
UniSource Energy Corporation.

(b)

Address of Issuer’s Principal Executive Offices
One South Church Avenue, Suite 100, Tucson, AZ  85701

Item 2.

(a)

Name of Person Filing

(b)

Address of Principal Business Office or, if none, Residence

(c)

Citizenship

This statement is being filed by White Mountains Insurance Group, Ltd., a Bermuda corporation (“White Mountains”), and its wholly-owned subsidiary White Mountains Advisors, LLC., (“Advisors”), a Delaware corporation (collectively the “Filing Persons”).  White Mountains is a property and casualty insurance holding company and Advisors is a registered investment advisor.

The address of the principal executive office of White Mountains is 80 South Main Street, Hanover, NH 03755.  The address of the principal executive office of Advisors is 370 Church Street, Guilford, CT 06437.

(d)

Title of Class of Securities
Common Stock (no par value)

(e)

CUSIP Number
909205106

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

(a)

o

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b)

o

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c)

o

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d)

o

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C 80a-8).

(e)

ý(1)

An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

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An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

ý(2)

A parent holding company or control person in accordance with §240.13d-1(b)(1)(ii)(G);

(h)

o

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

o

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

o

Group, in accordance with §240.13d-1(b)(1)(ii)(J).

Item 4.

Ownership

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a)

Amount beneficially owned:    

2,293,100*** shares.

(b)

Percent of class:   

6.7%.

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote   

0.

(ii)

Shared power to vote or to direct the vote    

2,293,100***.

(iii)

Sole power to dispose or to direct the disposition of   

0.

(iv)

Shared power to dispose or to direct the disposition of   

2,293,100***.

***         The reporting person directly controls no UniSource Shares and indirectly controls, through various wholly-owned subsidiaries and certain of its employee benefit plans (as further described herein), 2,293,100 UniSource Shares.

Item 5.

Ownership of Five Percent or Less of a Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following   o.

Item 6.

Ownership of More than Five Percent on Behalf of Another Person

Not applicable.

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person

Not applicable.

Item 8.

Identification and Classification of Members of the Group

Not applicable.

Item 9.

Notice of Dissolution of Group

Not applicable.

Item 10.

Certification

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Date: February 10, 2005

WHITE MOUNTAINS INSURANCE GROUP, LTD.

by:

                   /s/

Name: J. Brian Palmer

Title: Chief Accounting Officer

WHITE MOUNTAINS ADVISORS, LLC.

by:

                   /s/

Name:

Mark J. Plourde

Title:

Chief Operating Officer