Mentoring and psychological contracts are related and important organizationally based social exchange relationships. Research indicates that social support from internal mentors might buffer the negative effects of psychological contract breach (PCB), but less is known about the possible negative influence that mentors might have on psychological contracts. This study investigates prot,g,s' perceptions of their mentors' failure to fulfill mentoring obligations and prot,g,s' perceptions of PCB. Because either the formality of the mentoring relationship or the supervisory status of the mentor might serve as additional signals of the mentor's status as an organizational agent, and, therefore, exacerbate the negative association between mentoring breach and PCB, both are investigated as potential moderating variables. A sample of 144 prot,g,s from various occupations completed a questionnaire assessing mentoring breach, PCB, job satisfaction and organizational commitment. Results indicate the relations between mentoring breach and the outcome variables of interest were mediated by PCB, and that the relation between mentoring breach and PCB is stronger in supervisory mentoring relationships. A similar result was expected, but not confirmed, for formal mentoring relationships. The current study provides evidence that the actions (or inactions) of mentors contribute to PCB and related negative outcomes. Such knowledge might encourage organizations to provide training to potential mentors in managing the mentoring relationship to minimize the probability of mentoring breach. The current study is the first to examine the potential negative effect that mentoring relationships might have on prot,g, psychological contracts.