Mortgage interest tax deduction question: I pay 1/2 of all expences
living in a home owned by my girlfriend. My name is NOT on the title
or the mortgage loan, but can we split the mortage interest deduction
on our separate tax claims?

Hi cha24:

I'm sorry to say that the short answer is "No, you can't split the
mortgage interest deduction."

Why? Well, the main reason is that the deduction is only applicable to
the person *legally liable* to make those payments (i.e., the person
whose name is on the mortgage and title).

Your Federal Income Tax
URL: http://www.unclefed.com/TaxHelpArchives/2002/Pub17/p17-53.html#T478
Quote: 
********************
You can deduct home mortgage interest only if you meet all the
following conditions.

You must file Form 1040 and itemize deductions on Schedule A (Form 1040).
 
You must be legally liable for the loan. You cannot deduct payments
you make for someone else if you are not legally liable to make them.
Both you and the lender must intend that the loan be repaid. In
addition, there must be a true debtor-creditor relationship between
you and the lender.

The mortgage must be a secured debt on a qualified home. (Generally,
your mortgage is a secured debt if you put your home up as collateral
to protect the interests of the lender. The term qualified home means
your main home or second home. For details, see Publication 936.)
*******************************

This is backed up by the following information direct from the IRS:

Home Mortgage Interest Deduction - 2004
URL: http://www.irs.gov/pub/irs-pdf/p936.pdf
Quote:
*****************************
More than one borrower. If you and at least one other person (other
than your spouse if you file a joint return) were liable for and paid
interest on a mortgage that was for your home, and the other person
received a Form 1098 showing the interest that was paid during the
year, attach a statement to your return explaining this. Show how much
of the interest each of you paid, and give the name and address of the
person who received the form. Deduct your share of the interest on
Schedule A (Form 1040), line 11, and print ?See attached? next to the
line.
****************************

So, if you have no Form 1098, then there's no deduction for you, I'm afraid. 

One possible loophole I found comes into effect if the house is set up
as a "cooperative housing corporation" - then each stockholder in the
corporation could deduct interest expense commensurate with their
holdings. See pages 7-8 from the IRS document above for more
information.

But, from your description of the situation, I don't think that applies. 

Search Strategy (on Google):
* "who can claim" "mortgage interest" "tax deduction"
* "mortgage interest" site:.gov

I hope this helps. 

websearcher

While there is no way to split the deduction through the IRS, if your
girlfriend agrees that you are entitled to benefit from the payments
you have made toward the mortgage: she could do her taxes and take the
full deduction, and then figure out how much her taxes would have been
if she had taken only half the deduction. The difference is the
benefit she is receiving from taking "your share" of the deduction.
She could then give you that difference in cash.