The present study is a secondary analysis of a randomized trial of brief motivational interventions (BMIs) for 198 college students sanctioned for alcohol-related violations of school policy (Carey, Henson, Carey, & Maisto, 2009). Using multivariate latent growth curve models, we evaluated theoretically-derived mediators of the observed BMI effect: motivation to change (readiness-to-change, costs and benefits of drinking), and drinking norms (injunctive norms for peers, and descriptive norms for friends, local peers, and national peers). Results provided partial support for mediation by changes in perceptions of descriptive but not injunctive norms, a pattern that varied by gender and norm type. We found no evidence of a mediating role for any of the motivational variables.