This study aims to further our understanding of the social and cognitive processes underlying conceptual change learning through argumentative discourse. We tested the effect of competitive (vs. collaborative) argumentative discourse style and belief of interaction with a human peer (vs. a computerized peer agent) on learning the concept of diffusion through interaction with a disagreeing peer. Peer confederate’s verbal behavior was tightly controlled to evoke argumentative discourse, holding content exposure constant but differing in rhetoric style. Students in the collaborative discourse style condition performed better. Moreover even though previous studies have reported that the belief of interaction with a human peer benefits learning in consensual settings, the opposite was found for a settings in which the partner critiques the learner’s own solutions: Students performed better when they believed they interacted with a computer agent (vs. with a human peer). Implications for theory as well as task design are discussed.