Claims of bias in other people are common, but when do we perceive bias in others? Some research has looked at the links between bias and trustworthiness, perceived ulterior motives, and close-mindedness. This thesis looks at the influence of a source’s certainty on the bias that audiences perceive. Before reading a fictional political campaign message about a race for local county commissioner, participants were presented with a statement with one of three levels of certainty about the message. In these statements, the communicator indicated either high certainty or uncertainty. In the third control condition, no indication of certainty was made about their position. Source certainty was not found to influence perceived bias in the scenario studied; however, source certainty did influence perceptions of the source’s credibility. Perceived credibility mediated the effects of source certainty on perceptions of argument quality