Few empirical studies of psychological contracts have examined the role of unions and unfulfilled psychological contracts. This study has developed and tested a model proposing that union instrumentality moderates the relationship between psychological contract breach (PCB) and psychological contract violations (PCVs), and has investigated the mediating effects of such violations on the relationships between PCB and union commitment and trust in management (n = 215). Moderated regression analyses revealed that there was a significant interaction between PCB and union instrumentality in predicting PCVs. Specifically, the positive relationship between PCB and PCVs was weakened among employees endorsing high union instrumentality. Further, PCVs were associated with increased union commitment and decreased trust in management. Analyses also revealed that PCVs partially mediated the relationships between PCB and union commitment and trust in management. The importance of the findings is discussed and recommendations for future research are provided.