It is well established that increasing attitude certainty makes attitudes more resistant to attack and more predictive of behavior. This finding has been interpreted as indicating that attitude certainty crystallizes attitudes, making them more durable and impactful. The current research challenges this crystallization hypothesis and proposes an amplification hypothesis, which suggests that instead of invariably strengthening an attitude, attitude certainty amplifies the dominant effect of the attitude on thought, judgment, and behavior. In 3 experiments, the authors test these competing hypotheses by comparing the effects of attitude certainty manipulations on univalent versus ambivalent attitudes. Across experiments, it is demonstrated that increasing attitude certainty strengthens attitudes (e.g., increases their resistance to persuasion) when attitudes are univalent but weakens attitudes (e.g., decreases their resistance to persuasion) when attitudes are ambivalent. These results are consistent with the amplification hypothesis.