Purpose: Most past research on formal mentoring has investigated its antecedents, outcomes and benefits with little attention given to what goes on inside the dyadic relationship. The purpose of this paper is to explore the types of mentor and mentee behaviours that are perceived as critical factors contributing to either a positive or negative mentoring experience for the mentee and the mentor. Design/methodology/approach: Concrete examples of "effective" and "ineffective" mentor and mentee behaviour were collected from the research participants using Flanagan's Critical Incident Technique (CIT). The obtained CIT data were analyzed using forms of open and axial coding. Variants of content analysis were then used for conducting a series of subsequent comparative analyses. Findings: From a total of 187 coded critical incidents the study identified 11 positive and four negative behavioural criteria of mentoring effectiveness as perceived from the mentee perspective, and nine positive and three negative behavioural criteria of mentoring effectiveness as perceived from the mentor perspective. Comparisons against "theoretical" and "best practice" models and taxonomies of positive and negative mentoring reveal varying degrees of overlap and commonality. Research limitations/implications: There are two main limitations. First, the number of research participants was at the bottom end of the typical sample range for qualitative research, which means the collection of critical incidents did not reach the point of data saturation. Second, the study explored the "start-up" and "ongoing" phases of the mentoring lifecycle but not the "end" phase. Originality/value: The findings provide new insights into mentor and mentee behavioural effectiveness within formal mentoring relationships, and thereby add to a sparse empirical knowledge base in this substantially neglected area of mentoring research. Also, they provide a foundation against which to compare and contrast future empirical research that may be conducted on perceived effective and ineffective mentor and mentee behaviours within formal mentoring relationships. (Contains 8 tables and 1 figure.)