What is the relationship between political stability, trust, and source effects on support for public policies? In this article, we examine how source type (and the trust respondents have in different sources) impacts support for new policies and the degree to which this impact is moderated by political stability. This article reports the results of a survey experiment administered simultaneously in Australia and Hong Kong in late 2020 examining source effects on public attitudes toward COVID-19 vaccination and testing policies. For each case, the analyses compare source effects between a lesser trusted source (government) and a more trusted source (medical scientists). The study also compares these effects between cases, contrasting an environment of continuing political stability (Australia) with one of decreasing political stability (Hong Kong). Australian respondents tended to have similar attitudes toward policies regardless of the source, while Hong Kong respondents looked significantly more favorably on policies put forward by medical scientists than those put forward by the government. The results show that source effects can be moderated by political context—a finding that holds implications for the design of comparative studies about public trust and political legitimacy in settings where policy development relies on scientific input. Copyright © 2022 The Author(s).