BACKGROUND: Simultaneous alcohol and cannabis use is prevalent among adults who drink alcohol and is associated with more negative consequences than use of either substance alone. Understanding reinforcement processes that maintain simultaneous versus single-substance use will highlight intervention targets specific to this pattern of use. In individuals' daily lives, we tested whether simultaneous use moments are associated with more affectively reinforcing outcomes compared to single use moments. METHODS: We used ecological momentary assessment with 6 +  daily reports for 14 days. Participants were 88 adults ages 18-44 who reported simultaneous use at least twice per week. Mean age was 25.22 years and participants were 60.2 % female. At each momentary survey, participants reported alcohol and cannabis use, affect, momentary coping and enhancement motives, and subjective appraisals of use. RESULTS: Simultaneously using cannabis during alcohol-use moments attenuated the increase in negative affect that accompanied coping-motivated drinking (anxious mood b = -0.11, 95 % CI = [-0.19, -0.02], p = .016; depressed mood b = -0.14, 95 % CI = [-0.23, -0.05], p = .003). Simultaneously using cannabis during alcohol-use moments attenuated the positive association between enhancement drinking motives and subjective drinking-contingent pleasure (b = -0.34, 95 % CI = [-0.50, -0.18], p < .001). CONCLUSIONS: Simultaneously using cannabis during alcohol-use moments altered the subjective effects of alcohol, whereas simultaneously consuming alcohol during cannabis-use moments did not alter the subjective effects of cannabis. Individuals may perceive that simultaneous cannabis use mitigates undesirable effects of coping-motivated drinking, thereby driving simultaneous use of cannabis alongside alcohol.