Cooperative learning experiences can contribute to mathematics education reform by stimulating student communication. Sixth grade student conversations were recorded on four occasions over a four month period when they were working in cooperative groups. The results indicated that routine compliance with the requirement to “explain” superseded authentic dialogues about mathematical ideas. Student conversations were influenced by the model of explanation exchanges emerging from the teacher's visits to groups. Teacher influence was mediated by students' past experiences. The findings suggest that teachers implementing reform should help students develop criteria for judging mathematical arguments and confront student conceptions directly to deepen debates.