This article explores the challenge presented to governments and the scientific establishment by physicists who campaigned internationally on behalf of their Soviet scientific colleagues in the early 1980s. Cold War science operated in a highly charged environment: while the work of scientists on both sides of the Cold War divide was sponsored and closely guarded by government and military agencies, scientists were also at the forefront of activist challenges to human rights infringements suffered by their colleagues. The article explores the motivations for and limitations of a moratorium on participation in scientific exchange with the Soviet Union launched by the California-based group “Scientists for Sakharov, Orlov and Shcharansky” (sos). It considers the ways that both professional identity and Cold War dynamics shaped this solidarity campaign. sos sought to build their activism on a transnational basis and worked closely with scientific colleagues in Europe to do so. They pitched a campaign that appealed beyond the university and national scientific laboratories to a broad range of people who identified as scientists. Unlike many contemporary scientific organizations, the sos leadership embraced the political nature of such activism. As a whole, this article shows how scientists navigated different political and scientific contexts when organizing support for their Soviet colleagues.