Background: Chronic pain is a globally widespread condition with complex clusters of symptoms within a heterogeneous patient group. Internet-delivered Acceptance and Commitment Therapy (IACT) has shown promising results in the treatment of chronic pain. How IACT is experienced by patients is less well known. Qualitative studies of patients' experiences are needed to further understand factors behind both engagement and negative effects. The aim of this study was to explore how IACT was experienced by chronic pain patients who had participated in a controlled trial. Methods:Through an open and exploratory approach this study aimed to investigate how IACT was experienced when delivered as a guided self-help program to persons with chronic pain. Eleven participants were interviewed over telephone after completing IACT.Results: Qualitative analysis based on grounded theory resulted in 2 core categories and 8 subcategories. In treatment: Physical and cognitive restraints, Time and deadline, Therapist contact, and Self-confrontation. After treatment: Attitude to pain, Image of pain, Control or Command, and Acting with pain. Individual differences as well as specific conditions of the treatment may explain variations in how the treatment was approached, experienced and what consequences it led to. Therapist guidance and deadlines for homework play complex roles in relation to autonomy and change.Conclusions: Adjusting treatment content and format based on participants' characteristics, such as expectations, motivation and restraints, might positively affect engagement, autonomy and change. Further research on attrition and negative effects of treatment might clarify what enables chronic pain patients to benefit from IACT.Trial registration: clinicaltrials.gov (NCT01603797). Registered 22 May 2012. Retrospectively registered.