Personality traits underlying both anxiety disorders and depression are more malleable than previously presumed. This study examined associations between changes in personality traits (i.e. negative affectivity and detachment) and alleviation of anxiety and depression symptoms following cognitive behaviour therapy (CBT). We hypothesized that decreases in negative affectivity would predict alleviation of depression and anxiety symptoms and decreases in detachment would predict decreases in depression and, to a lesser degree, anxiety symptoms. Data (N = 156) were collected in a randomized controlled trial comparing transdiagnostic and diagnosis-specific group CBT for patients with major depressive disorder, social anxiety disorder, panic disorder or agoraphobia. We assessed personality traits using the Personality Inventory for DSM-5 (PID-5) and symptoms with the Hopkins Symptom Checklist 25-item scale (SCL). Prediction was based on regression analyses. We found that decreases in negative affectivity predicted lower levels of depression and anxiety symptoms while decreases in detachment only predicted lower levels of depression symptoms. The findings substantiate current efforts to explicate the dynamic interplay between personality traits and symptoms and support the existing focus on targeting negative affectivity and detachment in therapy for anxiety disorders and depression. The trial is registered at clinicaltrials.gov (ID NCT02954731).