This study investigated the effects of peer collaboration anddiscourse patterns on conceptual change in the context of high-schoolstudents learning from incompatible scientific information. The domainof investigation was biological evolution. Participants included 108students from Grades 9 and 12 randomly assigned to four conditions: (1)peer-conflict; (2) individual-conflict; (3) peer-assimilationand (4) individual-assimilation. Students were asked to think aloud ordiscuss with their peers eight statements consisting of scientificinformation ordered in ways that either maximized or minimized conflict.Several measures of prior knowledge and conceptual change were obtained.Peer collaboration resulted in some mixed findings suggesting that peereffects may vary depending on collaborative interactions. In-depthanalyses of collaborative interactions indicated two discourse patterns:‘surface’ moves included rating, ignoring, rejecting, and patching toeliminate differences; ‘problem-centred’ moves involved problemrecognition, formulation of questions, and construction of explanations.Comparisons between successful and unsuccessful learners showedsignificant differences in their proportional use of surface andproblem-centred moves. External conflict did not lead to deeperdiscourse and more conceptual change; students may need toexperience meaningful conflict. These findings suggest why peercollaboration only works sometimes and indicate the importance ofhelping students to engage in productive discourse.