Objectives: Treatment outcome studies ought to assess the fidelity of their treatments, including treatment delivery, but practical guidelines and examples for this are lacking. Based on general recommendations in available literature, this study proposes and illustrates the design and application of a Method of Assessing Treatment Delivery (MATD) in a behavioral medicine trial comparing two treatments for chronic low back pain. Study Design and Setting: In designing MATD, two experts identified several feasible treatment elements. Agreement between the experts in classifying these elements into five categories (essential and unique, essential but not unique, unique but not essential, compatible, prohibited) was assessed. In applying MATD, treatment recordings were evaluated by two independent raters, who coded the (non)occurrence of MATD elements and who categorized each session as belonging to one of the two treatments. Results: MATDs content validity was supported by adequate agreement between the experts' classifications of the treatment elements. MATDs interrater reliability was good. Conclusion: Comprehensive illustrations of designing and applying MATD may encourage the verification of treatment delivery as a partial reflection of treatment fidelity in forthcoming treatment outcome studies.