The purpose of our study was to explore peer mentoring styles and examine their contribution to academic success among mentees. Data were collected as part of a comprehensive evaluation of a peer mentoring program. The sample consisted of 49 mentors (advanced students) who supported 376 mentees (first year students) in small groups. Indicators for peer mentoring styles were constructed using mentee assessments of mentoring functions and mentor quality, and unobtrusive data gathered in an analysis of online mentoring activities and a content analysis of the quality of the online mentoring activities. Using cluster analyses, three distinct mentoring styles were identified: Motivating master mentoring, informatory standard mentoring, and negative minimalist mentoring. Motivating master mentors were shown to have a positive influence on success in the mentoring program among those mentees who failed two preliminary exams. Implications for the training of peer mentors in higher education are discussed. (Contains 2 figures, 2 tables, and 1 note.)