The retention of newcomers is of vital relevance for initiatives developing Free Libre Open Source Software (FLOSS). Based on previous FLOSS research which repeatedly highlights the amount of knowledge developers build in the beginning of their project participation for their continuance and experiences from the organizational domain, we evaluate the use of mentoring as an appropriate knowledge transfer and retention strategy for FLOSS projects. Combining FLOSS and organizational literature, we develop our research model and hypothesize that (i) mentoring facilitates novices in their learning, which, in turn, increases their retention (ii) and (iii) that mentoring has direct effects on protégés? continued participation. The evaluation of 91 newcomers to the KDE project supports our hypotheses and finds a strong direct and indirect association between mentoring and novices? project retention. On the one hand our analysis shows that mentoring significantly increases novices? achieved level of knowledge after their ramp up period which, in turn, enlarges their project permanence. On the other hand, we find evidence that there is also a strong and significant direct association between mentoring and protégés' retention behaviour, which could be the result of the strong interpersonal relationship which is formed between mentors and protégés.