Short answer is likely no. If you have always reported your items of income and expense on the cash basis, i.e., income reported when recieved and deductions when paid, then that is a "method of accounting" for which you cannot now adopt the accrual method - income is reported when earned and expenses when all of the events which create the fact and amount of the liability are determined. It is very, very unlikely from the outset of you filing your tax returns that you elected the accrual method. So, you would reflect the mortgage deductions in the tax year you actually pay them.