We developed an online exam to diagnose students who are underprepared for college-level general chemistry and implemented a program to support them during the general chemistry sequence. This transition program consists of extended-length recitations, peer-led team-learning (PLTL) study groups, and peer-mentoring groups. We evaluated this program's impact on student performance in general chemistry using data from the fall semesters 2007, 2008, and 2009. We found that our transition program helped the underprepared students make significant gains in their course performance relative to other students when controlling for prior content knowledge and experience. PLTL did improve the performance of the underprepared students relative to other students in the lower 40% of the class. Inclusion of peer mentoring resulted in additional gains over the use of PLTL. Via surveys to students in the transition program, students agreed or strongly agreed that extended-length recitations and peer-mentoring groups improved their performance in general chemistry. This is important given the voluntary nature of our program. (Contains 1 table and 3 figures.)