•Situational and social context characteristics increase risks for simultaneous use.•Larger numbers of underage drinkers increase risks for simultaneous use.•Lack of adult supervision increases risks for simultaneous use.•Simultaneous use is associated with increased subsequent risks.•Alcohol may be the primary contributor to risks associated with simultaneous use. We investigated contexts of simultaneous use of alcohol and marijuana and the impact of simultaneous use on problems among adolescents. Ecological momentary assessment data were obtained over two weekends from 150 adolescents in California (47% female, M age=16.36years), using smartphone surveys administered early and late in the evening and again the following morning. We assessed whether, in what context, and with whom adolescents drank alcohol and used other substances over 3 evening hours. We assessed problems they experienced each evening on the following morning. Results showed that greater adult supervision in every context was associated with a 55% lower risk of simultaneous use (RRR=0.45, p≤.05). Contexts with no other underage drinkers were associated with 99% lower risk of simultaneous use (RRR=0.01, p≤.005). Each occasion of simultaneous use was related to 110% increase in the number of problems (IRR=2.10, p≤.005), with 83%, 221% and 311% greater odds of violence (OR=1.83, p≤.05), driving under the influence or riding with a drunk driver (OR=3.21, p≤.05), or being drunk (OR=4.11, p≤.005). Additional analyses showed that these problems may be attributed largely to the alcohol consumed in each context. Results demonstrate that it is essential to consider situational and social characteristics of substance use contexts to better understand adolescent simultaneous use of alcohol and drugs and problems.