This article examines the Mexican state's surveillance of Spanish political exiles. As the Mexican government publicly welcomed over 20,000 political refugees fleeing the Spanish Civil War (1936–9), its intelligence apparatus characterised anarchist and communist refugees as subversive threats to the Mexican nation. Despite these efforts, the Mexican secret police failed to prevent the emergence of new political bonds between the two countries’ popular classes. This article shows the consequences of the Mexican secret police's campaign against radical exiles while also highlighting instances in which Spaniards evaded the state's purview and contributed to revolutionary projects in Mexico, Latin America and Spain.