This study investigated risky sexual behavior in a sample of 209 gay and bisexual men. Using structural equation modeling, the mediating relations of substance use factors (expectations about the sexually enhancing effects of substance use and substance use during sex) between internalized heterosexism (IH) and sensation seeking and unprotected anal intercourse were examined. Expectations that substance use would enhance sexual activity mediated the relationship between IH and risky sexual behavior. Both substance use factors mediated the relationship of sensation seeking to risky sexual behavior. Greater sensation seeking was associated with greater expectations about the sexually enhancing effects of substances, which in turn was associated with greater unprotected anal intercourse. Unexpectedly, less sensation seeking was related to more substance use during sex, which in turn was associated with more risky sexual behavior. Overall, the variables in the model accounted for 66% of the variance in sexual behavior. (Contains 2 figures and 1 table.)