Evidence-based psychotherapies for posttraumatic stress disorder (PTSD), such as cognitive processing therapy and prolonged exposure (CPT/PE), greatly reduce suffering for veterans, but many veterans fail to complete treatment. Developing a theory-based understanding of adherence is necessary to inform interventions to improve treatment retention. We developed and tested a series of scales applying the theory of planned behavior (TPB) to CPT/PE adherence. The scales were administered in mailed surveys as part of a larger mixed-methods study of veteran adherence to PE/CPT. Surveys were sent to 379 veterans who were initiating CPT/PE across four U.S. Veterans Affairs (VA) hospitals and 207 of their loved ones. Subsequent session attendance and homework compliance were coded via a review of electronic medical records. We examined item-level characteristics, factor structure, and the convergent and discriminant validity of the resultant scales. The findings support four subscales: two related to attitudes (i.e., Treatment Makes Sense and Treatment Fits Needs), one related to perceived behavioral control over participation (i.e., Participation Control), and one related to perceived family attitudes about CPT/PE participation (i.e., Subjective Norms). Scale validity was supported through significant associations with theoretically relevant constructs, including intentions to persist in CPT/PE, rs =.19–.38; treatment completion, rs =.21–.25; practical treatment barriers, rs = −.19 to −.24; and therapeutic alliance, rs =.39–.57.