Functional Analytic Psychotherapy (FAP) is emerging as an effective psychotherapy for psychiatric clinical cases. However, there is little research demonstrating the process of change of FAP. The present study evaluated the introduction and withdrawal of FAP interventions on therapy-interfering verbal behaviors of two participants who were in therapy with the researcher. The single-case experimental design was A-B1-B2-C1-C2 (A = therapy without planned interventions; B1 and B2 = introduction of FAP, and C1 and C2 = FAP withdrawal) and occurred over 20 sessions for each client. Results demonstrated both intrasubject and intersubject replication of the effects of FAP interventions on the verbal behavior of clients and confirmed that the main mechanism of change involved in FAP is the direct shaping of behavior in session, within the context of the therapeutic relationship. (Contains 3 figures and 1 footnote.)