The alliance between therapist and patient was investigated using the Psychotherapy Process Q-Set (PQS), an instrument that quantitatively describes therapy sessions in a manner that captures the complexity of the therapy process. More specifically, the PQS was used to examine the treatment processes being assessed by observer ratings on the California Psychotherapy Alliance Scales (CALPAS). Three PQS factors were found to be associated with alliance: Patient-Therapist Interaction, Patient Capacity/Commitment, and Therapist Countertransference. In multiple regression analyses with scales constructed from these three factors, Patient-Therapist Interaction was found to predict alliance ratings, while the other two aspects of the treatment process did not account for any significant additional variance in the alliance ratings. Results suggest that Patient-Therapist Interaction plays a defining role in the alliance construct, as assessed by the observer version of the CALPAS.