Dialectical behaviour therapy (DBT)‐informed skills training for borderline personality disorder (BPD) aims at the development of specific emotion regulation skills in patients, particularly with regard to the regulation of problematic anger. While the effects of dialectical behaviour skills training have been shown, their processes of change are rarely examined. Neacsiu, Rizvi and Linehan (2010) found that patient's self‐reported use of emotion regulation skills was a mediator of therapeutic change in these treatments; however, they found no effect for problematic anger. From an integrative perspective on anger (Pascual‐Leone & Greenberg, 2007; Pascual‐Leone & Paivio, 2013), there are several forms of anger, varying in their degree of therapeutic productivity. The present add‐on randomized controlled trial included n = 41 patients with BPD (n = 21 DBT‐informed skills training versus n = 20 treatment as usual). The first study examined the outcome of the DBT‐informed skills training encompassing basic components of training in mindfulness, distress tolerance, interpersonal effectiveness and emotion regulation. Results showed that symptom reduction was significantly greater in the DBT‐informed skills training, compared with the treatment as usual. The second study used process assessment, for which all patient completers underwent a 50‐min‐long psychological interview both early and late in treatment, which was rated using the Classification of Affective Meaning States. DBT‐informed skills training produced increased levels of primary ‘assertive’ anger, as compared with the treatment as usual, whereas no effect was found for ‘rejecting’ secondary anger. Most importantly, we showed that changes in assertive anger mediated the reported symptom reduction, in particular in patient's social roles. We discuss these results in the context of underlying mechanisms of change in DBT skills group treatments, in particular towards developing more productive forms of anger in this patient population. Copyright © 2015 John Wiley & Sons, Ltd. Key Practitioner Message A 20‐session dialectical behaviour therapy (DBT)‐informed skills training is a promising adjunct intervention for patients with borderline personality disorder, in particular for reducing problems related to social role. Increases in assertive anger mediate the effects of DBT‐informed skills training, whereas rejecting anger remains unchanged over the course of treatment. Short‐term objectives for intervention might involve the specific increase of assertive anger in BPD, by using DBT‐informed skills training; long‐term objectives for intervention might involve a specific decrease of rejecting anger in BPD.