Rousseau (1989 and elsewhere) argued that a defining feature of psychological contract breach was that once a promise had been broken it could not easily be repaired and therefore that the effects of psychological contract breach outweighed those of psychological contract fulfillment. Using two independent longitudinal surveys, this paper investigated whether changes in breach and fulfillment differentially affected work-related attitudes and affective well-being outcomes. Results generally supported differential effects, where increases in reported breach had greater effects on outcomes than increases in fulfillment, for the outcomes of affective wellbeing, job satisfaction, and organizational commitment. The present study underlined the importance of breach as a determinant of negative affect and attitudes. However, fulfillment had a lesser impact, in that it maintained (i.e., had no effect) or had only a modest positive effect on wellbeing and attitudes. (Contains 4 tables.)