I am currently in the US on a G4 visa as dependent of my wife who works for the UN. My wife's contract will end in January and we have to switch visa's then. I am entitled to a J1 visa as a researcher. On the G4 I have been filing taxes as a resident, and could get exemptions for my wife and two children. On the J1, I will be considered a non-resident and am unable to get exemptions for my wife and two children, as far as I understand. (The children are US citizens by the way.) Can I separately file my taxes as a resident for the part of the year I was on the G4, and then file as a non-resident for the rest of the year?

You can probably file as a resident for 2010 by CHOOSING to be treated as a resident for the entire year.

However, your J-1 visa MAY allow you to be exempt from U.S. taxes entirely. It depends on what the tax treaty (if there is a tax treaty) between your country and the U.S. says.

Please identify your home country.

I'm Dutch. I believe I'm exempt from tax for 2 years, but if I stay longer than that, or switch to another visa (either will be likely) I have to pay the taxes. Also my info tells me that I am considered a non-resident on a J1.

You are correct; whether you claim the tax-exempt status or not, you ARE considered to be a non-resident alien.

Also, there is a two-year limit for the exempt status, and, unfortunately, there is a retro-active tax provision if you exceed he two-year period.

Thanks for your reply! So if I have a J1 on the end of a tax year, I'll be considered a non-resident for the whole year, although I will have the G4 up to February?

Yes, that is correct.