Since the new conceptualization of motivation as a product of interpersonal exchanges, clinicians and researchers have shown a growing interest in understanding the process of and identifying factors increasing motivation for treatment. Elaborated from the self-determination theory, a motivational model was tested among 140 participants (recruited in a public outpatient readaptation center for substance-related disorders) who persevered in treatment for at least 6 weeks. The results support the usefulness of the self-determination theory in the field of motivation for treatment of substance-related disorders, and suggest some recommendations to enhance motivation: therapists must pay attention to the progression toward goal attainment and foster the perception of competence of their clients; treatment centers should offer a context that supports autonomy and that involves significant others.