Objectives: Mindfulness-based treatments have demonstrated efficacy in reducing symptoms in clinical populations. Not surprisingly, research suggests increases in client mindfulness might be a mechanism of change in these treatments. However, little is known about specific factors that lead to increased mindfulness. Design: The present study is a secondary analysis of 93 adults in outpatient treatment for substance abuse, assessing effects of between-session mindfulness practice and therapeutic alliance on levels of mindfulness after an 8-week Mindfulness-Based Relapse Prevention (MBRP) program. Results: Between-session practice over the course of the 8 weeks was predictive of mindfulness at postcourse, although not at the 2-month or 4-month follow-up assessments. Client-rated therapeutic alliance was a significant predictor at the 2-month follow-up, although not at 4 months. Conclusions: These findings suggest that between-session practice and therapeutic alliance might be important factors in the initial increases in mindfulness after mindfulness-based treatments, but factors supporting longer term mindfulness might shift over time.