The pollution due to textile dye effluents has been a major concern and the investigations involving various efficient techniques are being carried out to overcome this problem. This study presents the degradation of Congo red, an azo dye, using ultrasound and the degradation process was optimized by response surface methodology (RSM). The dye solution was subjected to ultrasound irradiation of 30 kHz at various initial concentrations and pH. The results showed that the initial dye concentration and pH of the dye solution influenced the % decolorization and low initial values resulted in high % decolorization. The toxicological studies were carried outwith bioluminescent assay indicated that the resulting products of sonolytic degradation were non-toxic. The production of oxidative species during the degradation was analyzed spectrometrically using iodometry and Fricke dosimetry. The degradation of Congo red was also confirmed by spectrograms obtained at different time intervals. Kinetic studies were carried out to determine the rate of the reaction and the influencing parameters such as pH and initial concentration were optimized using response surface methodology (RSM). (C) 2009 Elsevier B.V. All rights reserved.