Posttraumatic Stress Disorder (PTSD) and Substance Use Disorder (SUD) are highly comorbid, and comorbidity increases risk for poor functional outcomes. SUDs are associated with poor functional outcomes, such as quality of life, community engagement, and suicide (Teeters, Lancaster, Brown \& Back, 2017). Risks for poor quality of life and suicide increase further for those with co-occurring PTSD and SUD diagnoses as compared to either condition alone, with suicide attempt rates three times higher for Veterans with alcohol use disorder and PTSD (Norman, Haller, Hamblen, Southwick \& Pietrzak, 2018). For patients with PTSD-SUD, there is evidence of greater PTSD symptom severity and poorer SUD treatment outcomes (e.g., Back et al., 2000), as well as higher rates of homelessness and disability (Bowe \& Rosenheck, 2015). PTSD-SUD treatments have shown promising reductions in PTSD and SUD symptoms (Flanagan, Korte, Killeen \& Back, 2016). Yet, there are still major challenges in widely implementing concurrent or single-target gold-standard treatments for this population, especially with rural veterans where care access may be limited (e.g., Flanagan et al., 2016). Written Exposure Therapy (WET) is a front-line, brief and effective treatment for PTSD that addresses some of the challenges posed by other gold-standard treatments. This project is designed to examine the feasibility and acceptability of Written Exposure Therapy (WET) delivered to Veterans with comorbid PTSD-SUD while they are completing a 28 day-residential SUD program (DOM SUD). The preliminary effects of the treatment during the program, and at one month and 3-month follow-up periods will also be examined, with particular attention to rates of substance use, homelessness, treatment attendance, treatment completion, quality of life, suicidality, and PTSD and depression symptoms. Veterans enrolled in the DOM SUD with PTSD Checklist-5 (PCL-5) scores over 33 will be recruited for further screening into the study. Those that meet criteria for PTSD through confirmation using the Clinician Administered PTSD Scale for DSM-5 (CAPS-5) will be randomized into one of two treatment arms: Treatment as Usual (TAU: DOM SUD) and Written Exposure Therapy in a residential SUD program (resWET). Randomization will be stratified by gender and oversampling for women and minority participants will be used. Those in the TAU control group will participate in the DOM SUD treatment program, while those in the resWET group will also have five individual treatment sessions of WET. Participants will complete weekly measures of symptoms, in addition to rating cravings for substance use. Treatment completion rates will also be compiled for both DOM SUD and resWET. Participants will complete pre-treatment, post-treatment, 1 month, and 3 month follow-up measures in person, over video, or by phone, and these measures, along with data available from the treatment record, will be examined using descriptive analyses to look for important trends regarding both symptom responses (e.g., PTSD, depression) to treatment, as well as behavioral and functional outcomes (such as suicide attempts, homelessness, treatment attendance, treatment completion, substance use, quality of life). This preliminary data will be used to inform future studies using this treatment approach. Additionally, providers in the DOM SUD will complete treatment feasibility and satisfaction surveys and focus groups to provide essential information about implementation barriers that need to be addressed for the uptake of the treatment approach and to enhance the accessibility of the treatment. All Veterans will also provide feedback about their treatment, through survey when the treatment is complete, and focus group participation. Findings from both qualitative and quantitative data will be used to improve the treatment and assessment approach and to determine effect sizes of key measures (World Health Organization- Disability Assessment Schedule (WHO-DAS), PTSD Checklist (PCL), Patient Health Questionnaire (PHQ-9)) to prepare for a fully powered randomized control trial (RCT) to systematically evaluate resWET.