Co-use of alcohol and cigarettes is common and associated with greater negative consequences compared to use of either substance alone. Furthermore, alcohol and cigarettes are often used at the same time, and these "simultaneous" use events are associated with greater consumption of each substance. Given the prevalence and negative consequences associated with this pattern, we sought to identify proximal predictors and reinforcers of simultaneous use in individuals with a range of emotional and behavioral dysregulation who may be at greater risk of experiencing substance-related problems. Specifically, 41 adults who drank alcohol and smoked cigarettes (28 with borderline personality disorder and 13 community individuals) completed 21 days of ecological momentary assessment (EMA). First, we used multilevel models on cigarette-use moments to examine whether momentary cigarette motive endorsement differed based on whether participants were also drinking alcohol in that moment. Second, we used multilevel models on all EMA moments to examine whether simultaneous use was associated with greater craving and reinforcing effects compared to use of either substance alone. Participants reported greater enhancement and social motives for smoking cigarettes when also drinking alcohol compared to when they were only smoking. Participants also reported greater alcohol craving, greater sedation, attenuated positive affect, and greater fear following simultaneous use compared to use of either substance alone. Our results add to a growing body of research characterizing proximal influences on simultaneous substance use. Findings highlight potential treatment targets for individuals seeking to better understand or cut down on their use of alcohol, cigarettes, or both. (PsycInfo Database Record (c) 2022 APA, all rights reserved).