Client speech in favor of change within motivational interviewing sessions has been linked to treatment outcomes, but a causal chain has not yet been demonstrated. Using a sequential behavioral coding system for client speech, the authors found that, at both the session and utterance levels, specific therapist behaviors predict client change talk. Further, a direct link from change talk to drinking outcomes was observed, and support was found for a mediational role for change talk between therapist behavior and client drinking outcomes. These data provide preliminary support for the proposed causal chain indicating that client speech within treatment sessions can be influenced by therapists, who can employ this influence to improve outcomes. Selective eliciting and reinforcement of change talk is proposed as a specific active ingredient of motivational interviewing. (Contains 7 tables and 2 figures.)