AbstractBackground:Internet-based cognitive behaviour therapy (ICBT) is a promising format for treating different psychiatric disorders. In addition, several clinical trials have found positive results when using it to target transdiagnostic processes, such as perfectionism. However, few qualitative investigations have been conducted on the experiences of clients undergoing such treatments.Method:In the current study, clients completing 12-week guided ICBT for perfectionism responded to open-ended questions at post-treatment. In total, 30 out of 62 (48.4%) described their impressions of its content and the support provided by their guide.Results:The results were analysed qualitatively using thematic analysis. Five themes were found in the responses: Learning how to do things differently, Noticing the positives, Feeling safe to be honest, A comfortable treatment format and Barriers to treatment.Conclusions:The results suggest that many clients were able to achieve a change in perspective in relation to their perfectionism and started facing their fears. They were also able to report the benefits of doing things differently as part of treatment, such as an improvement in their interpersonal relationships. Most clients were also positive about the treatment format, enjoying its flexibility and the encouragement offered by their therapist. However, obstacles such as conflicting commitments, personal difficulties, time-consuming and comprehensive treatment modules, and a desire for more support were brought up by some, suggesting that there are aspects that could be considered in the future.