Objective: The purpose of this study was to identify potential determinants of objectively measured physical activity in the "Learning to Enjoy Activity with Friends" (LEAF) study. Design: This study involved a quasi-experimental design and students (N = 116) were assigned to an intervention group (n = 50) or a comparison group (n = 66) for a period of eight weeks. Setting: Three secondary schools (grades 7-12) in New South Wales (NSW), Australia were involved in the study. Method: At baseline and immediately following the intervention, students wore pedometers for four consecutive days and completed questionnaires assessing potential determinants of physical activity. At baseline, participants were classified using existing step recommendations, as low-active (girls less than 11,000, boys less than 13,000) or active (girls [greater than or equal] 11,000, boys [greater than or equal] 13,000) and the effects of the intervention on potential determinants were assessed using these subgroups. Subgroups were compared at baseline using independent samples t-tests and intervention effects were compared at post-test using linear regression (controlling for baseline measures). Results: Although the intervention had a statistically significant effect on physical activity among individuals classified as low-active at baseline, the intervention did not impact upon potential determinants of physical activity. Conclusion: Short-term changes in physical activity identified in the LEAF intervention were not mediated by changes in hypothesized determinants. (Contains 6 tables and 1 figure.)