Given the substantial and increasing encroachment of trade agreements into almost every aspect of economic and social life, there is a pressing need for research that provides a more coherent framework for understanding the source and effectiveness of organised labour’s power and capacity to influence international trade policy. Taking the union protests against the General Agreement on Trade in Services (GATS) as a case study, this research uses core concepts derived from social movement theory to analyse the opportunities that existed for unions to influence these trade negotiations and their capacity to identify and take advantage of such opportunities. Importantly, it adds a power analysis designed to reveal the sources of power that unions draw on to take action. The research demonstrates that even where unions faced considerable constraints they were able to reframe trade issues in a way that built broad support for their position and to utilise opportunities in the trade negotiation process to mobilise resistance against the GATS and further liberalisation of services. The theoretical framework developed for the research provides conceptual tools that can be developed for improving strategic campaign planning and for analytical assessment of past campaigns. The theoretical framework developed for this research has potential for further application as an analytical and strategic planning tool for unions.