Objective: In this paper, we present the development of a new self-report questionnaire to assess a characteristic of clinical work that has not been considered in the literature: the mental (affective and rational) work that therapists do between clinical sessions. The resulting instrument is the Post Session Therapist Questionnaire. Method: After consulting the literature and conducting in-depth discussions of the dimensions to be investigated, we generated 40 items through brainstorming. We had 16 experts carefully examine the questionnaire's content validity at various stages and thus obtained 38 pertinent, clear, and unambiguous items. The experts also provided positive evaluations of the questionnaire's instructions and response mode. We then carried out a pilot study with 27 clinicians, who filled out the questionnaire twice, referring to 100 patients in each case. To assess the structural validity of the new instrument, we conducted an exploratory factor analysis using the session (N = 200) as the unit of analysis. Result: Three factors substantially supported the theoretical dimensions that we conceptualized in the first stage of the research: We called the factors Positive Regard, Comprehension, and Diagnostic Effort. These dimensions have good internal consistency. Conclusion: The study's results suggest that these clinical dimensions are meaningful in the development of clinical processes, which encourages further study of the psychometric properties of the questionnaire, with larger samples, so as to consider these results in relationship to the outcomes of the following sessions.