We examined the association of individual demographic and behavioral attributes, partnership (dyad) and social network characteristics with unprotected sex in the heterosexual dyads of IDUs in St. Petersburg, Russia. Of the individual-level characteristics female gender and younger age; and of the dyad-level characteristics sharing injecting equipment, social exposure to the sex partner ("hanging out with" or seeing each other daily), and both partners self-reporting being HIV infected were associated with unprotected sex. Although self-reported HIV discordant couples were less likely to engage in unprotected sex, it was reported in over half of self-reported HIV discordant relationships. This study highlights the intertwining of sexual risk and injecting risk, and the importance of sero-sorting based on perceived HIV status among IDU sexual partnerships in St. Petersburg, Russia. A combination of social network and dyad interventions may be appropriate for this population of IDUs, especially for IDUs who are both injecting and sex partners, supported by free and confidential rapid HIV testing and counseling services to provide a comprehensive response to the wide-spread HIV epidemic among IDUs in St. Petersburg.