As mindfulness-based interventions become increasingly widespread, interest has grown in better understanding which features of these treatments produce beneficial effects. The present study examined the relative contribution of mindfulness practice time and practice quality in predicting psychological functioning (negative affect, emotion regulation, quality of life, mindfulness). Data were drawn from a randomized clinical trial of mindfulness training for smokers and assessed outcomes at posttreatment (n = 43) and 5-month follow-up (n = 38). The intervention included instruction in mindfulness techniques targeted to smoking cessation and relapse prevention and was composed of 10 group meetings over 8 weeks. Data from 8 treatment groups were used. Mindfulness practice quality was measured weekly over the course of treatment, and multilevel modeling was used to estimate trajectories of change in practice quality. The measure of practice quality was shown to be valid and reliable, with change in practice quality predicting change in psychological functioning at both posttreatment (β = .31, 95% CI = [0.04, 0.56], p = .022) and follow-up (β = .45 [0.16, 0.73], p = .002), even when controlling for practice time. Practice time predicted outcomes at posttreatment (β = .31 [0.05, 0.57], p = .019) but not at follow-up (β = .16 [−0.14, 0.47], p = .293). Neither practice time nor change in practice quality predicted smoking abstinence at 1 month or 6 months postquit. Results support the importance of practice quality as a relevant aspect of mindfulness interventions.