This paper presents the theoretical and methodological basis of a therapist's verbal behavior category system that allows us to study clinical psychologists' language from a functional-analytic framework and with a rigorous observation method. The procedure to develop the coding system is explained in detail from a very early stage of exploratory observation, to the systematic observation through the use of The Observer XT software. An analysis of intra- and inter-rater reliability using the kappa coefficient and taking into account the factors that affect the values of Cohen's index was carried out. Results show high levels of observer accuracy (between approximately 87% and 93%) that justify the application of this category system to study therapists' verbal behavior in session.