Research has addressed what instructional conditions may inhibit or promote scientific argumentation. Little research, however, has paid attention to interpersonal factors that influence collaborative argumentation. The present study examines the ways interpersonal factors affected group dynamics, which influence the features of collaborative argumentation among three groups of students of different ability as they collaborated to explain a complex scientific question. We transcribed and coded videotapes of three groups of 7th grade students while they performed an online science investigation and explanation. Methods included contextual analysis, discourse mapping, and qualitative case comparison. The results suggested that the clear goal of task completion allowed a single member to dominate group discussions, which prevented substantive argumentation. Students were mainly concerned about task identification and talked less about the meaning of their data. Our analyses suggest that social conflict may promote more substantive argumentation. We discuss ways to support more sustained argumentation during collaboration.