The Acceptance and Action Questionnaire–II (AAQ-II) is a self-report measure designed to assess experiential avoidance as conceptualized in acceptance and commitment therapy (ACT). The current study is the first to evaluate the psychometric properties of the AAQ-II in a large sample of adults (N = 376) with mild to moderate levels of depression and anxiety who participated in a study on the effects of an ACT intervention. The internal construct validity and local measurement precision were investigated by fitting the data to a unidimensional item response theory (IRT) model, and the incremental validity of the AAQ-II beyond mindfulness, as measured by the Five Facet Mindfulness Questionnaire, was assessed. Results of the IRT analyses suggest that the AAQ-II is a unidimensional measure of experiential avoidance and has satisfactory reliability for group comparisons in mild to moderately depressed and anxious populations. Item functioning was found to be independent of gender and slightly dependent on age in this sample. Furthermore, the AAQ-II showed incremental validity beyond 5 mindfulness facets in explaining depression, anxiety, and positive mental health. This study suggests the AAQ-II shows promise as a useful tool for the measurement of experiential avoidance in mild to moderately depressed and anxious populations.