An introductory conceptual and empirical review stresses the need for a stable theoretical basis for union commitment research. The purpose of this paper is to develop a new conceptualization of union commitment based on the integration of two theories (the theory of reasoned action and the rationalistic approach to commitment). The integrated theory suggests that union commitment is composed of two dimensions, one based on instrumentality and one based on ideology, which are causally related to pro-union behavioral intentions and, in turn, to union participation. Propositions derived from the integrated theory are tested using data on 1486 blue-collar workers in Sweden. Results of linear structural equation modelling with latent variables and of multiple regression analyses provide strong support for the construct validity of the commitment dimensions and the overall applicability of the integrated theory. The central findings, their conceptual implications for the understanding of union commitment, and their practical implications for unions are discussed.