["Scandinavian Journal of Psychology, Volume 67, Issue 2, Page 605-619, April 2026. ", "\nABSTRACT\nThis study's primary aim was to evaluate the efficacy of an internet‐based Acceptance and Commitment Therapy (IACT) program modified to include interoceptive exposure for treating panic disorder with or without concurrent agoraphobia. Its secondary aim was to examine whether therapist‐ and client‐rated working alliances were related to treatment outcomes. This randomized controlled trial included 79 participants, assigned to either a treatment group (n = 40) or a waitlist control group (n = 39) over 10 weeks. The study investigated the effects on panic disorder and quality of life, as well as the relationship between working alliances (rated by therapists and clients) and treatment outcomes. At post‐treatment, there was a significant between‐group treatment effect on panic disorder symptoms, with an observed effect size of d = 0.92. The model‐predicted effect size based on the multilevel model was dGMA‐raw = 0.86. Furthermore, 43% of participants no longer met the diagnostic criteria. Participants with concurrent agoraphobia exhibited higher initial panic symptom scores and were less likely to be diagnosis‐free post‐treatment. However, they still experienced significant and large treatment effects, with an observed effect size d = 1.22 and dGMA‐raw = 0,99. There was no significant between‐group difference in quality‐of‐life measurements. The therapist‐rated working alliance was associated with treatment outcome, but no significant relationship was found for the client‐rated alliance. Overall, the study suggests that interoceptive exposure‐modified IACT is an effective treatment for panic disorder and shows promise for patients with concurrent agoraphobia.\n"]