Objective: Recently, there has been increased interest in the study of emotion regulation difficulties in psychopathology. Anxiety disorders are among the most common mental illnesses, and growing research suggests that emotion regulation difficulties function as a maintaining factor in these disorders. A common characteristic of the various types of anxiety disorders is the use of inhibited emotion regulation strategies such as worry, suppression, and avoidance. Hence, our understanding of the etiology of anxiety disorders may benefit from embracing emotion regulation as a transdiagnostic factor. Traditionally, cognitive behavioural therapies (CBT) have attempted to treat anxiety disorders by targeting specific diagnoses. However, this approach has not proven sufficient for the understanding and treatment of chronic and comorbid forms of anxiety disorders. Transdiagnostic approaches to anxiety disorders emphasise the common processes of the illness and therefore have the potential to address multiple coexisting disorders simultaneously. This longitudinal study investigates how traditional diagnosis-specific CBT and a transdiagnostic version of metacognitive therapy (MCT) impact emotion regulation difficulties in anxiety disorders. We hypothesise that these cognitive treatments overall improve emotion regulation difficulties, and that the effect of transdiagnostic MCT is significantly larger than CBT. Methods: The participants of this RCT-study were recruited from the Department of anxiety disorders at Modum Bad in Norway. The study is part of a PhD project where the authors have worked as research assistants and participated in the collection of data. The participants were randomised into two different treatment groups (transdiagnostic MCT or CBT). The Difficulties of Emotion Regulation Scale (the DERS) was used as measure of emotion regulation difficulties and was conducted by the participants at pre-, post-, and 12-month follow-up. Multilevel models were used to analyse the data. Results and conclusion: The analyses revealed that both treatment conditions resulted in significant improvement in emotion regulation abilities. This finding suggest that problems stemming from difficulties with emotion regulation decrease as a result of cognitive-oriented therapies. Further, we found that treatment condition did not moderate changes in emotion regulation difficulties. Future research on how cognitive-oriented therapies affect emotion regulation should include process measures to better understand what mechanisms are involved.