Objective: To examine how relational qualities, including commitment to a sexual partner, are associated with condom use among young heterosexual adults at increased risk for sexually transmitted infections. Guided by the investment model of commitment processes, we hypothesized that sexual partner commitment is a function of satisfaction with, alternatives to, and investments in the relationship. Commitment to a sexual partner is, in turn, associated with reduced perceptions of vulnerability to sexually transmitted infection acquisition, which results in lowered condom use intentions and use. Method: We tested the hypothesized model using data from the Project on Partner Dynamics (POPD), a 4-wave, 1-year longitudinal study featuring a Time 1 sample of 538 African American, Hispanic, and White young adult from East Los Angeles, California, who provided data on all their sexual relationships over the year. Results: Findings from hierarchical path models supported the hypotheses, with relational qualities significantly linked to condom use via commitment, perceived vulnerability to harm from partner and intentions to use. Conclusion: These findings have implications for improving the health of high-risk individuals, including suggesting the importance of raising awareness of relational qualities that may give rise to unsafe sexual practices.