The study examined degradation of the antibiotics amoxicillin, ampicillin and cloxacillin in aqueous solution by the photo-Fenton process. The optimum operating conditions for treatment of an aqueous solution containing 104, 105 and 103 mg/L amoxicillin, ampicillin, and cloxacillin, respectively was observed to be H2O2/COD molar ratio 1.5, H2O2/Fe2+ molar ratio 20 and pH 3. Under optimum operating conditions, complete degradation of amoxicillin, ampicillin and cloxacillin occurred in 2 min. Biodegradability (BOD5/COD ratio) improved from � 0 to 0.4, and COD and DOC degradation were 80.8 and 58.4, respectively in 50 min. Photo-Fenton treatment resulted in the release and mineralization of organic carbon and nitrogen in the antibiotic molecule. Increase in ammonia and nitrate concentration, and DOC degradation were observed as a result of organic carbon and nitrogen mineralization. DOC degradation increased to 58.4 and ammonia increased from 8 to 13.5 mg/L, and nitrate increased from 0.3 to 14.2 mg/L in 50 min. © 2009 Elsevier B.V. All rights reserved.