This study analyses the characteristics of members leaving a Spanish union federation – Catalonia branch of Workers’ Commissions (CCOO-Catalonia), together with their reasons for leaving using a variety of data sources. Our findings indicate that higher union attrition among members in instable employment (i.e. casual employment and low seniority). In general, union leavers confirm that their job situation is an important reason for leaving the union. We therefore conclude that efforts made by the union to retain members in vulnerable labor market positions are important in reducing high rates of union attrition in Spain.