In spite of its potential for learning, and in particular knowledge revision, argumentation on science concepts is neither easily elicited nor easily sustained. Students may feel uneasy critiquing and being critiqued, especially on complex science topics. We report on a controlled study that tested the role of 2 potential factors that may either relieve or aggravate some of these concerns: the partner's argumentative discourse style (disputative or deliberative) and belief in interaction with a human or a computer agent. Learners interacted in scripted, computer-mediated interactions with a confederate on their understanding of a scientific concept they had just studied (i.e., diffusion). They were led to believe they were either interacting with a human peer or with a conversational peer agent. The peer confederate's verbal behavior was scripted to evoke argumentative discourse, while controlling for exposure to conceptual content and the type of dialogue moves, but differed in argumentative discourse style (disputative or deliberative). Results show that conceptual understanding of participants in the deliberative discourse style condition was higher than that in the disputative condition. Further, even though previous studies have reported that the belief in human interaction benefits learning in consensual interactions, the opposite was found to be true in a setting of disagreement and critique: Belief in interaction with a computer agent resulted in higher conceptual learning gains, compared to belief in interaction with a human peer. Implications for theory as well as instructional design are discussed.