Impulsivity is a robust risk factor for alcohol use among emerging adults (i.e., 18 to 25), but significant gaps remain in our understanding of the way that impulsivity relates to alcohol harms. Most prior research has been limited to between-level differences; thus, within-person variability in impulsivity at the momentary level and its bidirectional association with alcohol use has not been examined. Consequently, the present research used a 14-day ecological momentary assessment (EMA) design to: (1) determine momentary impulsivity as a predictor of subsequent alcohol use and problems; (2) examine the influence of alcohol use on subsequent impulsivity; (3) test socio-cognitive mechanisms (motives, expectancies, and norms) as real-time mediators explaining the link between momentary impulsivity and alcohol use; and (4) test context and sex as moderators of the relationship between momentary impulsivity and alcohol use. Participants were 96 (63 women) heavy drinking, college students. The mean age was 19.80 (SD = 1.76) years. Participants completed a baseline questionnaire and 14 consecutive days of momentary reports sent in the morning, afternoon, and evening; participants also completed two user-initiated reports during a drinking occasion (i.e., at the beginning and the end of their drinking). Multilevel modeling results indicated that greater levels of impulsivity experienced during the day was not associated with alcohol use or problems experienced that night. For bidirectional effects, alcohol use was associated with greater impulsivity reported at the end of the drinking occasion and greater alcohol use predicted greater impulsivity the next morning. Multilevel structural equation modeling revealed significant within-person mediation, such that on days when individuals reported greater than usual impulsivity, they also reported greater enhancement motives, positive expectancies, and negative expectancies, which in turn, was associated with greater alcohol consumption. Coping motives and norms did not mediate the association between impulsivity and alcohol use. A peer drinking context and sex were not significant moderators of the link between impulsivity and alcohol use. Overall, this study was the first to examine the bidirectional relationship between impulsivity and alcohol use as well as mechanisms and moderators using an EMA methodology. Findings supported impulsivity’s conceptualization as a state construct, and fluctuations in momentary states of impulsivity may coincide with alcohol use behaviors. Thus, findings from the present study contributed to conceptual daily process models of drinking by identifying how alcohol behaviors unfold in the real world. Future research is needed to examine other potential within-person mechanisms that may underlie or factors that may impact the examined relationships