Union mergers have occurred since unions were first established. Research on this particular aspect of the union movement is well established in the UK and the US. However, there are few studies of Australian union mergers, despite the fact that many Australian unions took part in a 'merger boom' in the 1980s and 1990s. Two of the few Australian studies, Hocking (1996) and Campling and Michelson (1998), utilised resource dependency and strategic choice theories to ascertain the why and how of union mergers. However, these Australian studies, like their UK and US counterparts, cease with the completion of the merger and, consequently, there is little known of the post-merger operation of unions. How does the integration of the merger partners - with their traditions, structures and financial arrangements - occur? This thesis rectifies the gap in the literature by means of a qualitative, longitudinal study of the merger and post-merger activities of one of the largest and most prominent unions in Australia, the Australian Manufacturing Workers' Union (AMWU), as it moved towards complete amalgamation. The thesis examines the period 1995-2003. Four of the five pre-merger unions faced a serious and continued decline in membership and all faced mounting financial deficits. From 1995, the year the various mergers were completed, membership decline continued and financial resources dwindled further, providing the impetus for further and significant post-merger changes. The analysis demonstrates that, due to a continuing lack of financial resources, the AMWU leadership initiated a budgeting strategy which influenced the actions and changed the opinions of many of the Union's officials, guiding them towards accepting integration of the constituent divisions and near complete amalgamation. The thesis answers the questions of why and how a union moves from a negotiated federated structure towards amalgamation, post merger, with a particular focus on financial decision-making processes.