Degradation of azo dye acid red B (ARB) on MnO2 in the absence and presence of ultrasonic irradiation (sonication) has been investigated. The effect of initial pH, anions (Cl-, NO3-, SO42-), and saturated gas (argon/oxygen) has been studied. The experimental results show that decolorization of the dye on MnO2 is highly pH dependent, with increasing decolorization efficiencies at decreasing pH values. The SO42- and NO3- ions are observed to suppress the adsorption of ARB on MnO2 dramatically, suggesting competitive nature with the dye. Sonication accelerates decolorization and mineralization of the dye. And oxygen used as saturated gas is more favorable for mineralization of the dye than argon. The average particle diameter of MnO2 decreases from 47.5 to 3.19 mum after sonication. (C) 2003 Elsevier Science B.V. All rights reserved.