This study proposes a theory of the combined influence of corporate and endorser credibility. Participants assessed the credibility of the companies and spokespersons as well as their attitudes toward the ads and brands, and their intent to purchase the advertised product. The covariance matrix was subjected to a path analysis. The model fit the data, and the findings corroborated prior research indicating that both types of source credibility have an impact on attitudes and purchase intentions albeit a differential one. The results suggest that the Dual Credibility Model partially predicts and explains advertising effectiveness for these dual sources of credibility.