Introduction: This study focuses on the development and validation of the Mentoring Processes Assessment Questionnaire--MPAQ- designed to assess mentoring processes in peermentoring programs aimed at first-year university students. Method: Participants in the study were 354 first-year students from a broad set of degrees at the University of Valencia in the 2013-14 and 2014-15 access cohorts. The factorial structure of the MPAQ was analyzed using both Exploratory and Confirmatory Factorial Analyses. A multi-group analysis was performed on the MPAQ to examine the factorial invariance and test the equivalence/non-equivalence of its structure across the participants' sex and age. The reliability of the factorial solution was also tested. Lastly, concurrent validity was assessed by examining the MPAQ relationships with (a) students' satisfaction with the program and the processes of mentoring and, (b) different psychosocial variables related to the permanence in the studies evaluated through the College Persistence Questionnaire. Results: The one-dimensional structure of MPAQ was confirmed, as well as its factorial invariance across sex and age. The reliability obtained was also adequate. Regarding concurrent validity, significant and positive relationships with students' satisfaction with the program and the processes of mentoring, academic and social integration, institutional commitment and satisfaction with support services were found. Discussion or Conclusion: Results highlight the MPAQ as a valid and reliable instrument for assessing peer-mentoring processes in programs for first-year university students and its potential usefulness in predicting retention after the first year of university, given its significant relationship with the psychosocial and emotional dimensions directly related to this phenomenon.