This article aims to study the collective strategies and social networks of young Spanish emigrants in the European Union, paying special attention to their perceptions of and practices regarding working conditions, as well as their relationships with the trade unions and social movements. The article focuses on two case studies of migrant self-organisation networks: the Union Action Group of Berlin, Germany and the Solidarity Federation in Brighton, UK. On the basis of semi-structured interviews and document analysis, the article concludes that the existing gap between trade unions and migrant labour can, under certain circumstances, favour the emergence of solidarity networks which in part play the role of trade unions. We call this type of organisation interstitial trade unionism.