This research investigates the influence of source credibility on attitude certainty, referring to an individual's subjective confidence in his/her attitude. Results of a laboratory experiment (N = 220) show that low source credibility, compared with high source credi-bility, leads to greater attitude certainty. This relationship, however, only holds when the source is identified before message exposure and for people who are low in need for cognition. Theoretical and practical implications of the findings are discussed. ©2009 Wiley Periodicals, Inc.