Background: The current study explored whether higher-(up to 8 modules) versus lower-dose (3 or less modules) unguided internet Cognitive Behaviour Therapy for perfectionism (ICBT-P) was more effective, and the best method to promote higher dosage. Methods: Two sequential randomised ICBT-P studies were conducted with participants who self-identified as having difficulties with perfectionism; in the first participants (N = 51) received 3-module ICBT-P or wait-list, and in the second participants (N = 55) received fixed (asked to complete all 8 modules two per week over 4-weeks) or flexible format (after completing the first psychoeducational module, participants decided how many/in what order they completed the modules). We examined impact on our primary variables, perfectionistic concerns and standards, and secondary outcomes of negative affect, body image flexibility, and self-efficacy. Results: More modules were completed in the higher-(M modules = 4.36, SD = 3.29) versus lower-dose (M = 1.96, SD = 1.23) ICBT-P, d = 0.86 (95% confidence interval: 0.39, 1.34). The latter impacted perfectionism but not secondary outcomes; the former impacted all outcomes (except for self-efficacy), and withingroup effect size improvements were double in the high-compared to low-dose ICBT-P. There was no difference between the fixed and flexible formats in terms of the number of modules completed or impact. Conclusions: We can offer a patient-centred approach to ICBT-P that is effective, while suggesting completion of more modules can result in larger, more pervasive improvements. ANZCTR Trial Number: ACTRN12619000147189. K E Y W O R D S body image flexibility, fixed or flexible format, internet cognitive-behavioural therapy, negative affect, perfectionism

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