The current study tests individual differences in response to a mandated brief motivational intervention (BMI) for college student drinking. Participants consisted of 99 (45% female) students who were referred for violating campus alcohol policy. Within-subject analyses suggest that the BMI led to a significant reduction in frequency of drinking and alcohol-related problems at the 1-month follow-up, with a nonsignificant trend in reduction of quantity of drinking and no difference in maximum level of drinking. However, there was a statistically significant amount of variability in response to the BMI across frequency, quantity, maximum, and alcohol-related problems. Individual differences in self-regulation and the strength of the therapeutic relationship were used to predict variability in change associated with participating in the intervention. Lower levels of self-regulation predicted more alcohol-related problems at 1 month following the BMI. A stronger therapeutic relationship predicted lower frequency, quantity, and maximum level of drinking at 1 month following the BMI. At the 12-month follow-up, there was a significant decay in the efficacy of the intervention, with a statistically significant amount of variability across all outcomes. Lower self-regulation was associated with more problems at 12 months. Individual differences in the therapeutic relationship were not associated with decay of efficacy between 1 and 12 months across any of the outcome variables. Results are consistent with both etiological heterogeneity and common factors models of the efficacy of psychotherapy, and extend these models to explaining outcomes of brief alcohol interventions in students mandated to treatment.