Perceived source credibility is fundamental to effective science communication. Recent years, however, have seen science become polarized and expert authority erode, complicating the notion that credibility equals persuasion. Two experiments address this tension, focusing on subcomponents of perceived source credibility—expertise and trustworthiness—for expert (N = 474) and nonexpert sources (N = 411). By manipulating the order by which participants learn about the source’s position on a scientific topic relative to their qualifications, the findings delineate when and how source credibility cues influence science communication, introducing nuance to the idea that credible sources can effectively communicate about value-laden topics.