Using a randomized controlled effectiveness trial, we examined the effects of Project SUCCESS on a range of secondary outcomes, including the program's mediating variables. Project SUCCESS, which is based both on the Theory of Reasoned Action and on Cognitive Behavior Theory, is a school-based substance use prevention program that targets high-risk students. We recruited two groups of alternative high schools in successive academic years, and randomly assigned schools in each group to either receive the intervention (n = 7) or serve as a control (n = 7). Students completed surveys prior to and following the administration of the program, and again 1 year later. Although participation in Project SUCCESS significantly increased students' perceptions of harm resulting from alcohol and marijuana use, students in the control group reported greater increases in peer support. We also found conflicting evidence in two opposing trends related to students' perceptions of the prevalence and acceptability of substance use. Therefore, the effects of Project SUCCESS on substance use-related beliefs and behaviors must be considered mixed. (Contains 2 tables.)