Background: Despite increasing research on psychological flexibility (PF) and its importance to psychological health, only a few measures exist for assessing this construct, and they have shortcomings, particularly in construct validity.Methods: This study assessed the factor structure, construct validity, and predictive value of the Open and Engaged State Questionnaire (OESQ) on samples with panic disorder and/or agoraphobia (n = 120), panic disorder with agoraphobia (n = 46), and burnout (n = 85). Results:The confirmatory factor analysis verified the expected one-dimensional structure and found good internal consistency in all three samples. Analysis of the construct validity revealed correlations to pathology, personality traits, and total functioning; it also revealed discrimination of PF from neuroticism when measured with the OESQ. Furthermore, we found that PF predicted outcomes of symptomatology. Conclusions:Considering the criticisms and suggestions for improvement in the literature on assessing PF, the OESQ proved to be a valuable operationalization of this construct. With better discriminant and incremental validity compared to other questionnaires as well as unique predictive value, the OESQ is important for future research on PF and acceptance and commitment therapy (ACT).

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