This article presents an analysis of the characteristics that are associated with union membership duration using data from the membership registers of the largest Spanish trade union: Comisiones Obreras (CCOO, Workers’ Commissions). Making use of survival analysis techniques, the results indicate that the shortest membership durations and the highest risks of leaving are associated with workers with poor employment conditions, mainly youth and foreigners, as well as those in firms, economic sectors, and territories where the union has a rather weak presence. As workers in these situations represent the majority of both current joining and leaving rates, the article concludes that retention policies should focus on the early stages of union membership.