Objective: To investigate the efficacy and proposed processes of change of ACT in improving the outcomes of IBS. Methods: Fifty six 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 4 time points (assessment, pre-treatment, post-treatment, 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 6 month follow-up. Preliminary support for the treatment process proposed was also found.