The state of the art for treatment efficacy studies now requires manual guided treatments and tests of therapist adherence. This report provides findings regarding adherence assessment of therapists participating in an investigation of treatment matching in adolescent substance abusers. The Group Sessions Rating Scale (GSRS), a group-therapy process measure, was studied to determine its appropriateness for assessing group treatment of adolescents with a) substance use disorders (SUD), b) interrater reliability, c) internal consistency, and d) ability to discriminate the active ingredients of cognitive-behavioral therapy (CBT) from interactional therapy (IT). Interrater reliabilities were moderate to high, with those for CBT generally higher than those for IT. Internal consistency of CBT items was moderate, whereas those of IT were moderately high. Discriminability between the two treatment modalities was high. The frequency of active ingredients was generally therapy-specific: high for the relevant and low for the nonrelevant therapeutic modality items. The GSRS was found to be effective in the measurement of treatment process in adolescents with SUD.