This study investigated exam achievement and affective characteristics of students in general chemistry in a fully-randomized experimental design, contrasting Peer-Led Team Learning (PLTL) participation with a control group balanced for time-on-task and study activity. This study population included two independent first-semester courses with enrollments of about 600. Achievement was measured by scores on exams written by an instructor blind to student participation. Established instruments were used to assess changes in attitude to chemistry and self-concept as a chemistry learner. No differences were found in achievement, attitude, or self-concept for students who participated in PLTL vs. those who participated in documented alternative study activities. Overall, certain aspects of attitude and self-concept showed a slight but significant decline from beginning to end of semester, consistent with previous studies. Males have higher positive attitude and self-concept than females, and first-year students have higher positive attitude, self-concept, and achievement than non first-year students. In a quasi-experimental comparison of 10 other course sections over seven years, students who self-selected into PLTL showed stronger exam achievement than those who did not choose to participate. These findings suggest that past reports of improved student performance with PLTL may in part be a consequence of attracting students who are already motivated to take advantage of its value.