Degradation of azo dye Acid Red B (ARB) on MnO2 enhanced by ultrasonic irradiation has been investigated. The effect of initial pH, anions (Cl-, NO3-, SO42-), and saturated gases (argon/oxygen) has been studied. The decolorization efficiency of the dye on MnO2 was strongly pH dependent and increased with decreasing pH. SO42- and NO3- inhibited the adsorption of ARB over MnO2 dramatically, suggesting competitive nature for adsorption with the dye. Ultrasonic irradiation enhanced degradation of the dye both for decolorization and mineralization. The formation of H2O2 and the decrease of MnO2 particle size during ultrasonic irradiation were also discussed. (C) 2003 Elsevier B.V. All rights reserved.