Objective: Randomized comparisons of acceptance-based treatments with traditional cognitive behavioral therapy (CBT) for anxiety disorders are lacking. To address this gap, we compared acceptance and commitment therapy (ACT) to CBT for heterogeneous anxiety disorders. Method: One hundred twenty-eight individuals (52% female, mean age = 38, 33% minority) with 1 or more "DSM-IV" anxiety disorders began treatment following randomization to CBT or ACT; both treatments included behavioral exposure. Assessments at pre-treatment, post-treatment, and 6- and 12-month follow-up measured anxiety-specific (principal disorder Clinical Severity Ratings [CSRs], Anxiety Sensitivity Index, Penn State Worry Questionnaire, Fear Questionnaire avoidance) and non-anxiety-specific (Quality of Life Index [QOLI], Acceptance and Action Questionnaire-16 [AAQ]) outcomes. Treatment adherence, therapist competency ratings, treatment credibility, and co-occurring mood and anxiety disorders were investigated. Results: CBT and ACT improved similarly across all outcomes from pre- to post-treatment. During follow-up, ACT showed steeper linear CSR improvements than CBT (p less than 0.05, d = 1.26), and at 12-month follow-up, ACT showed lower CSRs than CBT among completers (p less than 0.05, d = 1.10). At 12-month follow-up, ACT reported higher AAQ than CBT (p = 0.08, d = 0.42; completers: p less than 0.05, d = 0.56), whereas CBT reported higher QOLI than ACT (p less than 0.05, d = 0.42). Attrition and comorbidity improvements were similar; ACT used more non-study psychotherapy at 6-month follow-up. Therapist adherence and competency were good; treatment credibility was higher in CBT. Conclusions: Overall improvement was similar between ACT and CBT, indicating that ACT is a highly viable treatment for anxiety disorders. (Contains 2 figures, 6 tables, and 11 footnotes.)