Objective: Caffeinated alcoholic beverage (CAB) use is associated with a range of substance-related problems. The majority of prior research on CABs is based on cross-sectional and retrospective reports, which do not account for intraindividual differences in use and may be subject to recall biases. The current research used a daily diary, within-subjects design to compare days where individuals simultaneously mixed alcohol with caffeine (CAB days) and days where individuals drank other types of alcohol (non-CAB days) on alcohol use outcomes. These relationships were tested further by examining the impact of the type of mixer (i.e., energy drink or cola-caffeinated). Method: Participants were 122 (73.8% women) heavy drinking, college student CAB users. Mean age was 20.39 (SD = 2.08) years. Students completed a baseline questionnaire and up to 14 consecutive daily surveys about the previous night’s drinking behavior. Results: Multilevel modeling results indicated that CAB days were associated with heavier alcohol use, regardless of mixer type. In addition, beyond amount of alcohol consumed the previous night and trait impulsivity, CAB days were linked with more alcohol-related problems, but only when the mixer was an energy drink. CAB days did not differ from non-CAB days on driving or sex after drinking. Conclusions: This study was the first to demonstrate the unique risks posed by simultaneously consuming caffeine and alcohol in a within-subjects, daily diary design. Future research investigating use patterns may benefit from the use of fine-grained approaches in order to provide information relevant for CAB prevention and intervention efforts.