This study is a randomized controlled trial comparing two treatments for patients with comorbid substance use disorder (SUD) and post-traumatic stress disorder (PTSD) conducted within two different routine clinical settings for the treatment of SUD, namely (a) a day-care and inpatient setting and (b) an outpatient setting. Participants will be randomly assigned to either (1) a combination of cognitive-behavioral treatment (CBT) for SUD and trauma-focused structured writing therapy, or (2) CBT for SUD alone. Randomization will take place separately for each setting. It is expected that the combined treatment is significantly more effective in reducing symptoms of SUD as well as PTSD than CBT for SUD alone. Outcome measures will be assessed at pre-treatment, post-treatment as well as 3 months, 6 months and 12 months follow-up.

Inclusion Criteria:

• Diagnosis of substance abuse or substance dependence according to DSM-IV
• Diagnosis of PTSD according to the DSM-IV or subthreshold PTSD (at least one re-experiencing symptom plus either three avoidance/numbing or two hyperarousal symptoms are present)
• 18 years or older
• Sufficient fluency in Dutch or English to complete treatment and research procedures

Exclusion Criteria:

• Severe psychiatric problems that may interfere with study participation or that require more intensive clinical care than can be offered in the present study (e.g., dementia, psychotic symptoms, depression with suicidal ideation, manic episode and borderline personality disorder)
• Receiving concurrent psychotherapy
• Participants using tranquilizers are required to terminate medication use before inclusion
• Participants using anti-depressant medication are required to stabilize medication use before inclusion.