Background: The assessment of therapeutic adherence is essential for accurately interpreting treatment outcomes in psychotherapy research. However, such assessments are often neglected. Aims: To fill this gap, we aimed to develop and test a scale that assessed therapeutic adherence to Cognitive Processing Therapy – Cognitive Only (CPT), which was adapted for a treatment study targeting patients with post-traumatic stress disorder and co-occurring borderline personality symptoms. Method: Two independent, trained raters assessed 30 randomly selected treatment sessions involving seven therapists and eight patients who were treated in a multicentre randomized controlled trial. Results: The inter-rater reliability for all items and the total score yielded good to excellent results (intraclass correlation coefficient [ICC] = 0.70 to 1.00). Cronbach's α was .56 for the adherence scale. Regarding content validity, three experts confirmed the relevance and appropriateness of each item. Conclusion: The adherence rating scale for the adapted version of CPT is a reliable instrument that can be helpful for interpreting treatment effects, analysing possible relationships between therapeutic adherence and treatment outcomes and teaching therapeutic skills.