Can I deduct a contribution to a traditional IRA if I am also contributing to a 401(k) in the same tax year?
If you are eligible to participate in an employer-provided retirement plan like a 401(k), your right to deduct an IRA contribution is phased out once your income reaches a certain level. Note this is true even if you choose not to participate.
This is from the IRS website for 2010:
"If either you or your spouse was covered by an employer retirement plan, you may be entitled to only a partial (reduced) deduction or no deduction at all, depending on your income and your filing status. Your deduction begins to decrease (phase out) when your income rises above a certain amount and is eliminated altogether when it reaches a higher amount. These amounts vary depending on your filing status."
Here is the link: https://www.irs.gov/forms-pubs/about-publication-590-a