Introduction: Behavioral support improves smokers' chances of quitting, but quit rates are typically lower for smokers supported by "community practitioners" for whom smoking cessation is a small part of their job than for those supported by "specialist practitioners" for whom it is the main role. This article examined the factors that might contribute to this. Method: A total of 573 specialist practitioners and 466 community practitioners completed a 42-item online survey that covered demographic and employment information, current practices, levels of training, and 4-week CO-verified quit rates. Responses were compared for community and specialist practitioners. Mediation analysis was undertaken to assess how far "structural" and "modifiable" variables account for the difference in quit rates. Results: Specialist practitioners reported higher 4-week CO-verified quit rates than community practitioners (63.6% versus 50.4%, p < .001). Practitioners also differed significantly in employment variables, evidence-based practices, and levels of training. Six "modifiable" variables (proportion of clients using an "abrupt" quit model, duration of first session, always advising on medications, number of days training received, number of sessions observed when starting work, and number of sessions having been observed in practice and received feedback) mediated the association between practitioners' role and quit rates over and above the "structural" variables, explaining 14.3%-35.7% of the variance in the total effect. Conclusions: "Specialist" practitioners in the English stop-smoking services report higher success rates than "community" practitioners and this is at least in part attributable to more extensive training and supervision and greater adherence to evidence-based practice including advising on medication usage and promoting abrupt rather than gradual quitting.