ABSTRACT. Objective: Illicit drug and heavy alcohol use is more common among sexual minorities compared with heterosexuals. This difference has sometimes been attributed to more tolerant substance use norms within the gay community, although evidence is sparse. The current study investigated the role of perceived drug availability and tolerant injunctive norms in mediating the linkage between minority sexual orientation status and higher rates of prior-year substance use. Method: We used data from the second California Quality of Life Survey (Cal-QOL II), a followback telephone survey in 2008-2009 of individuals fi rst interviewed in the population-based 2007 California Health Interview Survey. The sample comprised 2,671 individuals, oversampled for minority sexual orientation. Respondents were administered a structured interview assessing past-year alcohol and illicit drug use, perceptions of perceived illicit drug availability, and injunctive norms concerning illicit drug and heavier alcohol use. We used structural equation modeling methods to test a mediational model linking sexual orientation and substance use behaviors via perceptions of drug availability and social norms pertaining to substance use. Results: Compared with heterosexual individuals, sexual minorities reported higher levels of substance use, perceived drug availability, and tolerant social norms. A successfully fi tting model suggests that much of the association between minority sexual orientation and substance use is mediated by these sexual orientation-related differences in drug availability perceptions and tolerant norms for substance use. Conclusions: Social environmental context, including subcultural norms and perceived drug availability, is an important factor infl uencing substance use among sexual minorities and should be addressed in community interventions. (J. Stud. Alcohol Drugs, 73, 675-685, 2012) A CCUMULATING EVIDENCE SHOWS THAT individuals with minority sexual orientation, regardless of their gender, tend to have higher rates of illicit drug and heavy alcohol use than do their same-gender heterosexual counterparts (Burgard et al., 2005;Cochran et al., , 2004Drabble et al., 2005;Gruskin et al., 2001;Hughes et al., 2010;McCabe et al., 2009;McLaughlin et al., 2010;Talley et al., 2011). Several population-based studies have reported higher rates of illicit drug and alcohol use and problem drinking among homosexually active women compared with exclusively heterosexual women, even after controlling for differences in sociodemographic characteristics (Burgard et al., 2005;Cochran et al., , 2004Drabble et al., 2005;Gruskin et al., 2001;Hughes et al., 2010;McCabe et al., 2009;McLaughlin et al., 2010;Talley et al., 2011). Further, a population-based survey of women ages 18-29 in low-income neighborhoods in northern California found that women who reported having both male and female sexual partners had signifi cantly higher rates of injection drug use compared with others (Scheer et al., 2002). Similarly, studies conducted with m...