Will I become responsible for my fiancé owed back child support to his exwife if I marry him?

    You are thinking ahead!  You are not responsible for child support  for a child that is not yours.  However, if you co-mingled accounts, assets and liabilities, it is sometimes difficult to distinguish between what would be your fiance's income/assets and yours.  Therefore, if you keep things separate, you should be okay.  But don't let him use your accounts to "shield" money, because this could create a problem for you.     One more important point:  The IRS can seize a tax refund that you may be entitled to if you file your income taxes jointly---even if that refund is "yours" because you paid in too much. Two solutions:  file separatedly (which usually creates a heavier taxable rate) or file jointly, but make sure you haven't paid in too much during the year that would entitle you to a refund. They can't seize what's not there. I suggest you check with your tax professional because there can be penalties if you don't pay in enough each year.  Each case is different so a CPA can assist you with this. Best of luck, Cindy Vova