Objective: The relationships between the occupational, educational, and verbal-cognitive characteristics of health care professionals and their motivational interviewing (MI) skills before, during, and after training were investigated. Method: Fifty-eight community-based addiction clinicians (M = 42.1 years, SD = 10.0; 66% Female) were assessed prior to enrolling in a 2-day MI training workshop and being randomized to one of three post-workshop supervision programs: live supervision via tele-conferencing (TCS), standard tape-based supervision (Tape), or workshop training alone. Audiotaped sessions with clients were rated for MI skillfulness with the Motivational Interviewing Treatment Integrity (MITI) coding system v 2.0 at pre-workshop and 1, 8, and 20 weeks post-workshop. Correlation coefficients and generalized linear models were used to test the relationships between clinician characteristics and MI skill at each assessment point. Results: Baseline MI skill levels were the most robust predictors of pre- and post-supervision performances. Clinician characteristics were associated with MI Spirit and reflective listening skill throughout training and moderated the effect of post-workshop supervision method on MI skill. TCS, which provided immediate feedback during practice sessions, was most effective for increasing MI Spirit and reflective listening among clinicians with no graduate degree and stronger vocabulary performances. Tape supervision was more effective for increasing these skills among clinicians with a graduate degree. Further, TCS and Tape were most likely to enhance MI Spirit among clinicians with low average to average verbal and abstract reasoning performances. Conclusions: Clinician attributes influence the effectiveness of methods used to promote the acquisition of evidence-based practices among community-based practitioners. (Contains 1 figure and 3 tables.)