While drinking motives are well-established proximal predictors of alcohol use, less is known about their role in event-level drinking behavior. The present study examines whether the interaction between individuals' drinking motives and the number of friends present at a given moment can predict alcohol consumption over the course of the evening. Using the Internet-based cell phone-optimized assessment technique (ICAT), 183 young adults (53.0 % female, mean age = 23.1) in French-speaking Switzerland completed cell phone questionnaires every Thursday, Friday, and Saturday evening over five weekends. A total of 7205 questionnaires completed on 1441 evenings were analyzed. Drinking motives and gender were assessed at baseline, while the hourly alcohol consumption rate and number of friends present were assessed at 8 p.m., 9 p.m., 10 p.m., 11 p.m., and midnight. Multilevel growth curve models with time-invariant and time-varying covariates were estimated for men and women separately. Among women, enhancement motives were associated with an increase in the hourly alcohol consumption rate over the course of the evening (b = .025; p < .05). The impact of the number of friends present on the hourly consumption rate was stronger among those women who scored high on coping motives at baseline (b = .028; p < .05). Among men, drinking motives were found to have no moderating effects. Results highlight the role of drinking motives and their interactions with situational characteristics in determining event-level drinking, especially among women. Strategies to prevent risky weekend drinking should focus on both the social environment in which drinking takes place (e.g., the drinking group) and individual drinking motives.