This RCT investigates whether Cognitive Processing Therapy, a first line treatment for PTSD, can be enhanced by the use of the odor molecule Hexadecenal (HEX). HEX is secreted from body odor and has been shown to promotes psychological well-being and positive social interaction. For the active group Hex will be diffused in the treatment room during all treatment sessions. For the control group a vehicle placebo molecule will be diffused. Both therapists and patients will be blind to study condition.

In this triple blind (patient, therapist, independent evaluator) RCT, 40 patients with PTSD receiving standard Cognitive Processing Therapy will be randomized in a 1:1 ratio to receive either odorless Hexadecanal molecules or odorless mineral oil molecules (Control) diffused in the treatment room at all therapy sessions. Inclusion criteria: age\>18 years, PTSD diagnosis according to DSM-5-TR. Exclusion: Bipolar, psychotic, or neurological disorders, drug abuse, any parallel psychotherapy, anosmia. Primary outcome: pre-to-post treatment change in PTSD symptom severity measured by the PTSD check List for DSM-5 (PCL-5). Additional outcomes: Depression (PHQ-9), anxiety (GAD-7), Anger (DAR-5), therapeutic alliance (WAI-SF).