A psycholinguistic account of motivational interviewing (MI) is proposed. Critical to this view is the assumption that therapists and clients are natural language users engaged in a constructive conversation that reveals and augments relevant information about the status of future change in a clients substance abuse. The role of client speech acts-most notably, verbal commitments-during MI is highlighted. How commitments can be signaled in client speech or gestures is discussed. How these commitment signals can inform therapeutic process and subsequent behavioral outcome is then put forth. Using natural language as a measure, a MI process model is presented that not only posits a mediational role for client commitment in relating underlying factors of desire, ability (self-efficacy), need, and reasons to behavior, but also a pivotal role as a need-satisfying enabler of a social-cognitive mechanism for personal change.