Dole and Sinatra (1998) have argued that promoting the level of engagement necessary to facilitate conceptual change could be achieved by “weighing issues and arguments” (p. 121). The present study sought to test the efficacy of a conceptual change intervention based on argumentation. Participants who were asked to argue in favor of an alternative explanation of a physics problem (the scientific explanation) were more likely to show improved reasoning on that problem than control participants who were asked to solve the problem without argumentation. We argue that this intervention is consistent with other pedagogical techniques that promote conceptual change (such as conducting experiments and benchmark lessons, diSessa & Minstrell, 1998, or generating self-explanations, Chi, 2000), in that it fosters high engagement and opportunities to juxtapose ideas, with the added advantage of ease of implementation and significant effects with little investment of instructional time.