Objective. This study examined socioeconomic disparities in adolescent substance use utilizing a behavioral economic theoretical framework. We tested the hypothesis that teens of lower (vs. higher) socioeconomic status (SES) are vulnerable to substance use because they engage in fewer pleasurable substance-free activities that provide reinforcement and may deter substance use. Method. In a cross-sectional correlational design, 9th grade students (N = 2839; mean age = 14.1 years) in Los Angeles, California, USA completed surveys in Fall 2013 measuring SES (i.e., parental education), alternative reinforcement (engagement in pleasurable substance-free activities, e.g., hobbies), substance use susceptibility, initiation, and frequency, and other factors. Results. For multi-substance composite outcomes, lower parental education was associated with greater likelihood of substance use initiation in the overall sample, frequency of use among lifetime substance users, and susceptibility to substance use in never users. Substance-specific analyses revealed that lower parental education was associated with higher likelihood of initiating cigarettes, alcohol, and marijuana use as well as greater susceptibility to use cigarettes in never smokers. Each inverse association between parental education and substance-related outcomes was statistically mediated by diminished alternative reinforcement; lower parental education was associated with lower engagement in alternative reinforcers, which, in turn, was associated with greater substance use susceptibility, initiation, and frequency. Conclusion. These results point to a behavioral economic interpretation for socioeconomic disparities in adolescent substance use. Replication and extension of these findings would suggest that prevention programs that increase access to and engagement in healthy and fun activities may reduce youth socioeconomic health disparities related to substance use. (C) 2015 Elsevier Inc. All rights reserved.