This study examined (a) differences between alcohol-only users and alcohol-marijuana co-users and (b) motives for use in relation to alcohol and marijuana use and problem use. Spring 2016 data among 1,870 past 4-month alcohol users (63.6% female, 69.1% White) from seven Georgia colleges/universities were analyzed cross-sectionally and with regard to problem use measured 4 months later. Correlates of co-use (n = 345; vs. alcohol-only use, n = 1,525) included greater alcohol and marijuana use frequency, problem drinking and marijuana use, and alcohol use motives (p's < .05). Controlling for covariates, alcohol use frequency correlated with greater marijuana use frequency and Coping and Self-enhancement alcohol use motives, but lower Conformity alcohol use motives (p's < .001); greater Coping and Self-enhancement alcohol use motives (p's < .01) predicted problem alcohol use. Marijuana use frequency correlated with greater Coping and Expansion marijuana use motives (p's < .05); greater Expansion marijuana use motives (p = .005) predicted problem marijuana use. College-based substance use interventions should target Coping and Self-enhancement alcohol use motives and Expansion marijuana use motives.