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Simultaneous alcohol and marijuana (SAM) use is prevalent among young adult drinkers and associated with increased risk for harms. Less understood about SAM use is whether increased risk is incurred on SAM use occasions relative to occasions in which individuals used only 1 substance. From a sample of young adult SAM users, we compared occasions in which individuals simultaneously used alcohol and marijuana so that the effects overlapped (“SAM days”), occasions involving only alcohol (“alcohol days”), and occasions involving only marijuana (“marijuana days”) on level of use, level of subjective intoxication, and negative and positive consequences. Eligible participants (N = 154; 57.8% female, 72.7% non-Hispanic/Latinx White) were 18–25 years old, reported past-month SAM use, and reported past 2-week binge drinking. Participants completed up to 14 daily assessments on their substance use behavior. Multilevel models indicated that, relative to alcohol days, SAM days had more negative consequences. SAM and alcohol days were not statistically different on level of use, level of subjective alcohol intoxication, or positive consequences. Relative to marijuana days, SAM days had more negative consequences and more positive consequences. Level of use (number of hits) and level of marijuana intoxication were not statistically different. In addition to SAM users being a more at-risk group, SAM occasions are associated with greater risk within this vulnerable population. Fine-grained ecological momentary assessments are needed to better understand perceptions and behavior throughout a SAM use episode.