Many researchers and clinicians believe that the therapeutic relationship is essential in bringing about clinical change. Empirical research to support this contention is scarce in part due to the difficulty of specifying and measuring theoretically derived mechanisms of change and the important dimensions of the client-therapist relationship. Functional Analytic Psychotherapy (FAP; Kohlenberg & Tsai, 1991) is a behavioral treatment that delineates how the therapeutic relationship brings about clinical change in clear and measurable terms. While initial research has been conducted to demonstrating the effectiveness of FAP with different populations, the purported mechanism of clinical change in FAP has not been sufficiently documented. This study describes the creation of a behavioral coding system (the Functional Analytic Psychotherapy Rating Scale; FAPRS) to identify and specify the components believed to be essential in bringing about client behavior change in FAP. Interobserver agreement values indicated moderate to high levels of reliability for the coding system. Implications for future tests of FAP's proposed mechanism of change and the validity of the coding system are discussed. (Contains 7 tables.)