Dying is not an excuse for not paying income tax. If your spouse had taxable income the last year of his life, the IRS expects a return. The money to pay the tax bill comes out of his estate. The responsibility for making out the return usually falls on the estate executor.
The Income
Typically your spouse and your spouse's estate each file a tax return. Income your spouse received before he dies goes on his 1040; income for the remainder of the year belongs to the estate. If your spouse filed returns on a cash basis, like most people, only money that was actually paid to him before he died counts as income. If he used accrual accounting, money he earned but hadn't been paid yet has to be added.
Joint Return
If you file a joint return, it includes your spouse's income up to the time of her death and your income for the entire year. You work with the executor to crunch the numbers, and both of you sign the return. If there isn't time to get an executor involved -- if the 1040 for the death year is due with the IRS and an executor hasn't been appointed yet -- you can make out the return by yourself.
Paying
You don't have to pay your spouse's share of the tax bill, or the estate's income tax. The executor takes care of paying both bills, using the estate's cash and assets. If money is tight and he has to choose which bills to pay, the taxman comes ahead of other creditors. If you file a joint return and the estate can't cover your spouse's final tax bill, the IRS can come after you. Both parties on a joint return are liable for all of the tax.
Innocent Spouse
If you sign a joint return and your spouse wasn't entirely honest on his taxes, the IRS may hit you after his death with demands for money he under-reported or hid from the government. You can apply to the IRS for "separation of liability" -- a ruling that you're not responsible for your dead spouse's tax debt -- provided you didn't know about the fraud or error he committed. If the IRS accepts, you're off the hook.
