Background: Few studies have examined why smoking cessation interventions are effective. The aim of this study was to examine the mediating processes underlying the effectiveness of cessation counseling administered by the Dutch national quitline. Methods: Data were used of a two-arm randomized controlled trial in which smoking parents, who were recruited through primary schools in The Netherlands, received either quitline cessation counseling (n = 256) or a self-help brochure (n =256). The endpoint was 6-months prolonged abstinence at 12-months follow-up, with 86.7% outcome data retention. Putative psychological mediators of treatment effectiveness included smoking-related cognitions (positive smoking outcome expectancies, self-efficacy), emotions (negative affect, perceived stress, depressive symptoms), and smoking cue coping methods (avoidance coping, acceptance coping) assessed at 3-months post-measurement. Results: Quitline cessation counseling significantly decreased positive smoking outcome expectancies and negative affect and increased self-efficacy to refrain from smoking, avoidance of external cues to smoking, and acceptance of internal cues to smoking compared to self-help material. Increased self-efficacy to refrain from smoking in stressful and tempting situations (p