Background and aimsAlcohol and marijuana are widely used among college students. Emotion regulation strategies have been linked to alcohol and marijuana use, but little attention has been devoted to modeling the directionality of these associations. The aims of the current study were to test whether (a) daytime use of emotion regulation strategies influences the likelihood of evening substance use and (b) evening substance use influences the likelihood of next-day use of emotion regulation strategies. DesignLongitudinal daily diary data were collected for 30days via on-line surveys. SettingNortheastern United States. ParticipantsA total of 1640 college students (mean age=19.2years, 54% female, 80% European American) were recruited each semester between Spring 2008 and Spring 2012. MeasurementsDaily diaries assessed emotion regulation strategies (distraction, reappraisal, problem-solving, avoidance) and substance use (any drinking, heavy drinking, marijuana use, co-use of any drinking/heavy drinking and marijuana). Covariates included gender, age, race/ethnicity, fraternity/sorority involvement and baseline depression. FindingsDaytime distraction [odds ratio (OR)=0.95], reappraisal (OR=0.95) and problem-solving (OR=0.94) predicted lower odds of evening marijuana use (P-values<0.02). Evening heavy drinking (OR=0.90) and marijuana use (OR=0.89) predicted lower odds of next-day problem-solving, with heavy drinking also predicting higher odds (OR=1.08) of next-day avoidance and marijuana use also predicting higher odds (OR=1.08) of next-day reappraisal (P-values<0.03). ConclusionsThere appear to be reciprocal relations among emotion regulation strategies and substance use: greater daytime use of distraction, reappraisal, and problem solving predicts lower evening substance use, while higher evening substance use predicts higher next-day avoidance and reappraisal but lower next-day problem-solving.