This article provides insights into the dynamics of the professionalization of trade union officials. Using the example of the Fédération Chimie-Énergie, which is affiliated with the Confédération française démocratique du travail (FCE-CFDT), this qualitative research, based on interviews and archival documents, investigates the gradual nature of the professionalization of union officials, the role expectations that they face and the problematic nature of their professional reintegration at the end of their mandate. While the professionalization of trade unions is conceived of as an answer to the problems of internal coordination and of the legitimacy of trade union action, the article shows that it engenders new organizational dilemmas. All in all, the article contributes to the knowledge of the professionalization of social movements and, more broadly, to the sociology of collective action.