This study focuses on the implementation of a peer-led team learning (PLTL) instructional approach for all students in an undergraduate organic chemistry course and the evaluation of student outcomes over 8 years. Students who experienced the student-centered instruction and worked in small groups facilitated by a peer leader (treatment) in 1996–1999 were compared with students who experienced the traditional recitation section (control) in 1992–1994. Quantitative and qualitative data show statistically significant improvements in student performance, retention, and attitudes about the course. These findings suggest that using undergraduate leaders to implement a peer-led team learning model that is built on a social constructivist foundation is a workable mechanism for effecting change in undergraduate science courses. © 2002 Wiley Periodicals, Inc. J Res Sci Teach 39: 606–632, 2002