Following Friedman and Harvey's (1986) call for additional theory development of the union commitment construct, this study applies the theory of reasoned action (Ajzen & Fishbein, 1980; Fishbein & Ajzen, 1975) to the study of union commitment. The theory is tested using questionnaire data from diverse samples of 1,914 professional union members from Sweden and 1,088 blue-collar union members from the United States of America. Confirmatory factor analysis results of LISREL 7 indicate support for the two dimensions of union commitment posited by the theory - Union Attitudes and Opinions, and Pro-Union Behavioural Intentions. The construct validity of the dimensions is demonstrated through correlational data. The similarity of results across the diverse samples provides evidence of cross-cultural generalisability of the dimensions, and suggests support for the application of the theory of reasoned action to union commitment research.