Drawing on a study of423 union members in a UK Primary Care Trust, we use structural equation modeling to investigate the extent to which organizational and union commitment and citizenship behaviors are a function of economic and social exchange. Findings suggest that the organization-employee and union-member exchanges are distinct, with organizational commitment a function of organization-employee exchange and union commitment and citizenship behaviors (UCB) a function of union-member exchange. Findings also suggest that union commitment and UCB are motivated primarily by social exchange, with economic exchange playing a supporting role. In addition, for organizational commitment, social exchange also seemed to be relatively more important. There was no evidence that perceived industrial relations climate moderates these relationships. Implications for industrial relations and workplace social partnerships are considered.