Wastewater containing dimethyl sulphoxide (DMSO), a widely used organic solvent in the semi-conductor industry, is usually classified as an industrial waste requiring high-cost treatment. This study was conducted to evaluate the feasibility of the biological treatment of DMSO wastewater with activated sludge (AS). The optimum conditions for Fenton treatment were also investigated. The optimum chemical dosage of H2O2: Fe2+ for Fenton treatment was 1000:1000 mg/l for wastewater containing 800 mg/l of DMSO. Although TOC and COD removal efficiencies by Fenton treatment were not satisfactory for most applications, the BOD/COD ratio was increased from 0.035 to 0.87, suggesting it as a very useful pretreatment method for biological treatment. Wastewater containing 800 mg/l of DMSO was treated successfully by AS without Fenton pretreatment, after 20 days acclimation period. Fenton pretreatment or pre-acclimation with easily biodegradable organics did not significantly reduce the acclimation period. Average removal efficiencies of TOC, SCOD, and SBOD by AS at an HRT of 24 h (loading rate of 0.8 kg DMSO/m(3)-day) were 90%, 87%, and 63%, respectively. Most of the sulphur in DMSO was oxidized to sulphate, eliminating the possibility of the production of sulphide-containing noxious intermediates. For 3500 mg/l of DPS-1300 wastewater containing 1925 mg/l of DMSO, satisfactory effluent qualities were obtained by AS at an HRT of 72 h (loading rate of 0.64 kg DMSO/m(3)-day). Control of pH was an important operating factor for AS operation as protons are produced as a final product of DMSO degradation. Results indicated that DMSO wastewater can be successfully treated with AS, which may significantly reduce the treatment cost compared to the chemical methods currently used. (C) 2001 Elsevier Science Ltd. All rights reserved.