Abstract Psychological Inflexibility (PI) is a construct that has gained recent attention as a critical theoretical component of Acceptance and Commitment Therapy (ACT). PI is typically measured by the Acceptance and Action Questionnaire-II (AAQ-II). However, the AAQ-II has shown questionable reliability in clinical populations with specific diagnoses, leading to the creation of content-specific versions of the AAQ-II that show stronger psychometric properties in their target populations. A growing body of the literature suggests that PI processes may contribute to hair pulling, and the current study sought to examine the psychometric properties and utility of a Trichotillomania-specific version of the AAQ-II, the AAQ-TTM. A referred sample of 90 individuals completed a battery of assessments as part of a randomized clinical trial of Acceptance-Enhanced Behavior Therapy for Trichotillomania. Results showed that the AAQ-TTM has two intercorrelated factors, adequate reliability, concurrent validity, and incremental validity over the AAQ-II. Furthermore, mediational analysis between emotional variables and hair pulling outcomes provides support for using the AAQ-TTM to measure the therapeutic process. Implications for the use of this measure will be discussed, including the need to further investigate the role of PI processes in Trichotillomania.