This study aimed to evaluate the theories of Ajzen (Planned Behavior) and Triandis (Interpersonal Behavior) on influencing 698 junior high school students and 306 senior high school students at two sites in Quebec, Canada. Baseline questionnaires were completed as well as at 9 months of follow-up. Results show that compared to junior students in the control group, juniors in the intervention group had positively modified attitudes, greater perceived behavioral control, personal beliefs and role beliefs, anticipated intention to delay sexual intercourse, and use condoms and self-efficacy to negotiate these behaviors. Compared to seniors in the control group, seniors in the intervention group showed positive modification in the above variables, except perceived behavioral control and anticipated intention to delay sexual intercourse. These seniors were more likely to use condoms regularly also. A few differences were observed between male and female respondents. These findings suggest that theory-based approaches are effective in modifying psychosocial variables. Involvement of peer educators in program design increases their own behavioral modification.