Barcelona was the capital city of European anarchism during the inter-war years. The aim of this article is to discover the sociological and territorial features of the radicalized CNT (the Confederación Nacional del Trabajo), the anarchist union, which generated the summer 1936 revolution. By looking at the role of urban space as a variable in the collective processes of the working class the article argues that the unskilled recent immigrant worker and the neighbourhoods where this working-class figure was dominant were the key protagonists of revolutionary radicalism.