This study examines whether expectancies about the impact of not drinking or reducing alcohol use and perceptions of peer alcohol use partially mediated risk incurred by sensation seeking for adolescent alcohol involvement. High school drinkers (N = 3,153) completed a survey assessing substance use, sensation seeking, perceived peer alcohol use, and non-drinking expectancies. Single and multiple mediation models were used to estimate the size of the mediated effect. Participants' expectations about the consequences of not drinking explained a substantial portion of the relationship between sensation seeking and alcohol use for both males and females (43.9% and 22.7%, respectively). Perceived peer alcohol use was also relevant to this link for females (8.4%). The role of these potentially malleable cognitive processes in adolescent decision making regarding alcohol use have direct prevention and intervention implications for helping to reduce risky alcohol use among high-risk adolescents. (Contains 1 figure and 3 tables.)