Research on substance use motivestypically examines each substance separately. However, simultaneous alcohol and marijuana (SAM) use-thatis, using alcohol andmarijuanaatthe same timesothattheireffects overlap-iscommonamongyoungadults. This study examines day-today fluctuations in motivesfor using alcohol and/ or marijuana among young adult substance users as predictors of alcohol, marijuana, and SAM use across days. Method: Data were from a community sample of young adults whoreported SAM use in the past month (analytic sample: N =399,mean[SD]age =21.63 [2.17];50.9% women). Participants reported alcohol, marijuana, and SAM use, and also motives"for alcohol and/or marijuanause" for14consecutive days. Results: Multilevelmodels showedthat elevated enhancement motives were associated withheavy episodic drinking, drinkingmore, and more hours high from marijuana. Elevated social motiveswere associated with heavy episodic drinking and drinking more, and also with fewerh ours high.Elevated conformity motives were associated with drinking more. SAM usewas more likely:onalcohol days and on marijuana days with elevated enhancement and conformity motives, on alcohol days with elevatedc oping motives, and on marijuana days with elevated social motives. Conclusions: SAM useonagiven daywas primarily associated with enhancement and conformity motives. Social motivesw ere more stronglylinked to alcohol use, and to some extent coping motivesw ere linked to marijuana use in this young adult sample.Further examination of situation-specific motivesa nd contexts of use is needed to inform development of real-time interventions for SAM use and consequences.