Transnational collective agreements (TCAs) are an important development in the international dimension of industrial relations. This article compares four case studies of multinational companies in the UK covered by TCAs. Findings show that while the formal influence of TCAs was limited, they were invoked around particular disputes and could strengthen union influence in a context otherwise characterized by limited union rights. Such influence depended on the co‐ordination of workplace‐ and firm‐level industrial relations institutions, union access to management at headquarters level and union receptiveness to and outward engagement with transnational activity. The formal but also the informal dimensions of these dynamics played a significant role.