Psychological inflexibility and experiential avoidance are key constructs in the Acceptance and Commitment Therapy (ACT) model of behavior change. Wolgast (2014) questioned the construct validity of the Acceptance and Action Questionnaire-II (AAQ-II), the most used self-report instrument to assess the efficacy of ACT interventions. Wolgast suggested that the AAQ-II measured psychological distress rather than psychological inflexibility and experiential avoidance. The current study further examined the construct validity of the AAQ-II by conducting an online cross-sectional survey (n = 524), including separate measures of experiential avoidance and psychological distress. Confirmatory factor analyses indicated that items from the AAQ-II correlated more highly with measures of depression, anxiety, and stress than the Brief Experiential Avoidance Questionnaire (BEAQ). Implications include that, as broad measures of experiential avoidance, the AAQ-II and BEAQ may not measure the same construct. In terms of psychological distress, the BEAQ has greater discriminant validity than the AAQ-II, and perhaps an alternative instrument of psychological inflexibility might be needed to assess core outcomes in ACT intervention research.