Per diem is reimbursement to you and it only needs to be reported to the IRS if you itemize your deductions. It wouldn't need to be reported to the IRS if you took the standard deduction that goes for all employer reimbursements I believe. I wonder if you would do well to file a 1040x a year or two back and change from married filed together if you did to married filed sepersately and of course do that from now on until you are officially singe.