Recent research (Tormala & Petty, 2002) has demonstrated that when people resist persuasion, they can perceive this resistance and become more certain of their initial attitudes. This research explores the role of source credibility in determining when this effect occurs. In two experiments, participants received a counterattitudinal persuasive message. When participants counterargued this message, they became more certain of their attitudes, but only when it came from a source with high expertise. When the message came from a source with low expertise, resisting it had no impact on attitude certainty. This effect was shown using both a traditional measure of attitude certainty (Experiment 1) and a well-established consequence of certainty—the correspondence between attitudes and behavioral intentions (Experiment 2). In addition, the effect was confined to high elaboration conditions, and occurred even when participants were not explicitly instructed to counterargue. These results are consistent with a metacognitive framework proposed to understand resistance to persuasion.