In this study the antibiotic sulfamethoxazole (SMX) was subjected to ozone treatment. Solutions of 60 mg/L and 100 mu g/L SMX in pure water and secondary municipal effluent were treated. The removal profile of SMX and its oxidation products was monitored as a function of transferred ozone dose in both matrices. No difference was observed in the ozone dose required for the concentration of SMX to fall below the limit of detection in pure water and wastewater. New peaks with the same retention times were obtained on the HPLC chromatograms for all conditions studied. Solutions with an initial concentration of 60 mg/L required 83 mg/L of ozone to fall below the limit of detection and eight oxidation products were detected. Solutions with an initial concentration of 100 mu g/L required 14 mg/L of ozone and only four oxidation products were detected. The four peaks obtained during experiments at low concentration were observed at the same retention times as four of the peaks obtained in higher concentration samples. In ozonated wastewater these products were identified as: 4-aminobenzene sulfonamide, N-(3-phenylpropyl)-acetamide, 2-methyl-benzoxazole and phenol. In addition, methanol, ethanol, acetic acid, methyl acetate and ethyl acetate were identified in the higher concentration samples. (C) 2009 Elsevier B.V. All rights reserved.