Current literature has emphasized the lack of research into verbal behaviors of teachers as a barrier to understanding the effectiveness of instructional interventions. This study focuses on the verbal behaviors of peer leaders, who serve as de facto teachers in a college chemistry teaching reform based on cooperative learning. Video data obtained throughout a semester of General Chemistry I from two different peer leaders, each interacting with a different group of students, was subjected to two rounds of qualitative data analysis. First, Toulmin's argumentation scheme was used to characterize the arguments constructed by group members during peer leader intervention. Next, verbal behaviors exhibited by the peer leaders during intervention were examined. Findings of this study showed that peer leaders used an array of verbal behaviors to guide students to build chemistry knowledge, and that a relationship existed between student argument components and peer leader verbal behaviors, with data most frequently emerging in response to short questions from the peer leader, and warrants in response to probing and clarifying questions. The findings from this study have implications for professional development of teachers at all levels, specifically for demonstrating the interplay between group intervention strategies and student discourse within cooperative learning groups.