Alcohol expectancies are well-studied between-person risk factors for problem drinking. However, no studies have tested mechanisms through which daily deviations in expectancies relate to drinking behavior during acute drinking episodes. This study filled this void, testing a sequential mediation model regarding the roles of social context, subjective responses, and craving in relations between daily deviations in expectancies and drinking behavior. Participants ( = 131) who reported past-month binge, social, and solitary drinking completed 21 days of morning and afternoon ecological momentary assessments and event-contingent drinking reports. Multilevel models tested whether daytime expectancies predicted social context at drink initiation, which indirectly predicted within-session drinking through deviations in subjective responses and craving. Expectancies/subjective responses were measured across valence/arousal (high arousal positive/reward, low arousal positive/relaxation, high arousal negative/aggression, low arousal negative/impairment). Increased daytime expectancies predicted experiencing the expected effect while drinking, even when controlling for context and consumption. Increased daytime rewarding expectancies predicted initiating drinking in social contexts, which indirectly predicted heavier within-session drinking via increased rewarding subjective effects and craving. In contrast, daytime relaxation expectancies predicted lesser within-session drinking, above and beyond context, subjective effects, and craving. Finally, increased daytime aggression expectancies predicted aggressive subjective effects, which indirectly predicted heavier within-session drinking via increased craving. Expectancies regarding later drinking predicted context-specific drinking and subjective effects, consistent with self-fulfilling prophecies of alcohol effects. Future research should consider testing the efficacy of coupling daytime fluctuations in expectancies with adaptive interventions seeking to increase protective strategy utilization. (PsycInfo Database Record (c) 2025 APA, all rights reserved).