The aims of this study as to evaluate and understand the decolorization of azo dyes using carbonate and hydrogen peroxide under low-frequency ultrasonic irradiation. Under optimal conditions, the decolorization ratio of acid orange 8 (AO 8), a typical azo dye, was > 90% after 2 h of irradiation. The decolorization rate of AO 8 was 0.023 min(-1) under ultrasonic irradiation, which was about two times that without ultrasound. Different from the results of other published studies, (OH)-O-center dot played a minor role, while CO3 center dot- played the most important role in AO 8 ultrasonic decolorization in the presence of CO32- and H2O2, with a contribution of 56.52%, followed by CO42- (32.61%) and O-1(2) (10.87%). Another difference is that CO3 center dot- formed through the cleavage of peroxymonocarbonate or peroxydicarbonate under ultrasonic irradiation rather than through reaction between hydroxyl radical and carbonate. Investigations for different azo dyes revealed that the decolorization rate decreased in the order AO 8 approximate to orange II > acid red 9> acid yellow 11, probably because of molecular differences among the azo dyes. (C) 2017 Elsevier B.V. All rights reserved.