Union mergers raise central questions about a union's raison d'être and the range of constituents it can serve effectively. Whereas the challenges posed by union purpose and scope of membership have long engaged researchers, few studies have fleshed out the contradictory issues that engage committed members and leaders during a merger. We argue that in mergers, fundamental themes in unionism intersect in paradoxical ways. We analyse a merger of tertiary education unions in New Zealand and situate its debates within a changing context for industrial relations. The case provides insight into the tensions inherent in union mergers by addressing the drivers for integration and differentiation as well as the role of leaders in shaping these. The findings elucidate paradoxical dynamics inherent in mergers: the paradox of purpose confronts the tension between industrial and craft unionism and the paradox of membership scope considers critical mass and communities of interest. The conclusion reflects on identity, leadership and context during mergers and poses contributions to research and practice.