ABSTRACT. Objective: Although the concepts of harmonious and obsessive passion have been productive in explaining why people eagerly engage in such activities as sports, Internet use, and gambling, previous research has not yet extended these models to explain alcohol and marijuana use among college students. The current research was conducted to clarify the relationships among harmonious and obsessive passion, alcohol and marijuana use, and negative consequences. Method: Two studies were conducted using online assessments. In Study 1, 748 heavy drinking college students (58% female) were recruited and completed measures of passion for drinking alcohol, alcohol use, and alcohol-related negative consequences. In Study 2, 352 regular marijuana-using students (54% female) were recruited and completed assessments of marijuana passion, marijuana use, and marijuana-related consequences. Results:Study 1 found that among heavy drinking college students, harmonious passion was a stronger predictor of increased consumption than was obsessive passion, whereas obsessive passion was a stronger predictor of alcohol-related problems than was harmonious passion. Study 2 revealed similar findings with regard to harmonious passion predicting marijuana consumption; however, unlike Study 1, no significant difference between the passions was found in predicting marijuana-related problems. Conclusions: This research provides a novel perspective on motivation for alcohol and marijuana use. Findings suggest that understanding the locus of young adults' passion for substance use may be helpful in identifying those who are likely to develop a substance use disorder and therefore may be the most in need of assistance and intervention. (J. Stud. Alcohol Drugs, 76, 749-757, 2015)