Based on 10 weekly telephone interviews with first-year college students (N=202; 63% women; M=18.8 years, SD=0.4), within- and between-person associations of positive and negative affect with alcohol use were examined. Multi-level models confirmed hypothesized within-person associations between weekly positive affect and alcohol use: Higher positive affect weeks had greater alcohol consumption, more drinking and heavy drinking days in the same week, and less plans to drink the following week. However, between-person, average positive affect did not predict individual differences in alcohol use. The negative affect--alcohol use association was complex: Within-person, higher negative affect was associated with less drinking days but between-person, with more drinking days; lability in negative affect was associated with greater average alcohol use and more drinking and heavy drinking days. Health promotion efforts for late adolescent and emerging adult students are advised to recognize these paradoxical effects (e.g., promoting dry celebratory campus-events, strategies to manage negative mood swings).