Background: Social network support for abstinence has been associated with improved treatment outcomes among samples of individuals with alcohol use disorders. As a result, research studies have focused on the inclusion of significant others (SOs) in the treatment process. Nonetheless, little is known about 1) the specific influence SOs may have on clients during treatment sessions or 2) whether SO within-session behaviors have any relationship to client post-treatment drinking. Method: In the current study, Motivational Enhancement Therapy sessions in which a SO was present were coded using a behavioral coding system designed to measure SO and client within-session language. Results: Relationships were observed between SO and client within-session language. Furthermore, some specific SO categories of language predicted post-treatment client drinking. Conclusions: This study is the first systematic evaluation of SO contributions in substance abuse treatment sessions. Future research examining SO language in the treatment of alcohol use disorders might allow clinicians to avoid contributions from SOs that are associated with poorer drinking outcomes.