Don't believe you can force the issue. There may be some leverage by electing to itemize on your return. The code requires that both spouses itemize if either elects to do so. This means that your wife would have to itemize and would not be entitled to use a standard deduction which could affect the refund she is expecting. You would likely benefit from itemizing anyway depending on your situation.

If she agrees, you can still file a joint return.

Are you in a community property state? If so, since you were together you each need to report 1/2 of each others income. Explaining that to her might encourage her to change to joint.

Toni and Stephan raise good points. If you have to file as MFS and you can itemize it will cause her return to get kicked. Point this out to her. She may not have been entitled to the refund she got if she should have itemized.

You may also want to talk to your estranged spouse and see if you can BUY a joint return from her - let me explain.

The law will allow you to amend and go from MFS to MFJ. BUT since she already got a refund you'll have to motivate her to do so. The best motivation is money. Offer to give her some of the refund you'll get if you file MFJ and she what she says.

In the end, even if you give her ALL of the MFJ refund it would be better than you filing MFS and PAYING. I know it sucks, but you cannot force her to file MFJ - even if you were married and living happily ever after.

Good luck

It has nothing to do with who filed first. Joint filing is allowed only if you both agree. The only situation in which you can file jointly without your spouse's consent is if your spouse is deceased. There are no other exceptions.

Even if your spouse had not already filed separately, you would be required to file separately unless your spouse either died or agreed to file jointly.

Based on the facts as given i don't believe that you itemizing and costing her $84 ($4,500-$3,650 = $850 taxable income, MFS=$84) in taxes because she has no itemized deductions will have much impact. If she agrees to file jointly she is individually liable for the total taxes owed on that return, even though she only earned $4,500. It would be a difficult sell if she has an advisor.

If she has really already filed, there isn't a lot you can do. You can't file jointly, that is no longer an option. You could file separately and itemize, but that will mean your return will get looked at by the IRS and there will be an adjustment on either your return or hers.