--- - |2+ Background The aim of this study was to investigate the efficacy and proposed processes of change of acceptance and commitment therapy (ACT) in improving the outcomes of irritable bowel syndrome (IBS). Methods A total of 56 consecutive patients recruited from a specialist clinic were included in the study and completed an ACT treatment protocol (one‐day group workshop plus self‐help manual). Assessments of process (acceptance of IBS) and outcome variables (symptom severity, avoidance behaviours, quality of life, and gastrointestinal anxiety) were carried at four time points (assessment, pre‐treatment, post‐treatment, and follow‐up). Results A significant increase in the acceptance of IBS and improvement in all outcome variables was observed from pre‐ to post‐treatment and follow‐up (effect sizes medium to large). Improvements in all outcomes were associated with increases in acceptance of IBS. Changes in acceptance of IBS from pre‐ to post‐treatment were a significant predictor of improvements in outcomes from pre‐treatment to follow‐up. Conclusions Results support the efficacy of a brief ACT protocol in improving IBS outcomes and maintaining therapy effects at six‐month follow‐up. Preliminary support for the treatment process proposed was also found. - Clinical Psychologist, Volume 22, Issue 2, Page 241-250, July 2018.