This study explored the predictors, role and effects of motivation in chronic substance users as they began residential treatment, using the Self-Determination Theory (SDT) framework. Testing of the SDT model involved a sample of 350 clients from six therapeutic communities across Australia who completed a battery of standardized measures. It was hypothesized that clients with a more autonomous motivational attitude toward treatment would be more likely to engage in the therapeutic process and would report more psychological well-being in the initial stages of the therapeutic process. It was also anticipated that clients who felt supported in an autonomous fashion by therapeutic staff would be more autonomously motivated with regard to being in treatment. Findings provided support for the hypothesized effects.