Fenton's reagent (Fe2+/H2O2) is known as one of the most effective and most often employed advanced oxidation process (AOPs) for the treatment of textile dyes and dyehouse effluent. In the present study, a synthetic acid dyebath effluent (SADB) bearing two azo and one anthraquinone dye together with two dye auxiliaries was subjected to pretreatment with Fenton's reagent. Firstly, initial Fe2+ and H2O2 concentrations as well as pH were optimized to achieve highest COD and color removals during Fenton's treatment of SADB. In the second stage of the experimental work, kinetic studies were conducted to elucidate the effect of operating temperature (20 degrees C < T < 60 degrees C) on COD, color abatement and H2O2 consumption kinetics. Obtained results indicated that 30% COD and practically complete color removal (99%) could be achieved at T = 50 degrees C. The kinetic studies revealed that a strong correlation existed between COD removal and H2O2 utilization rates. In the final part of the study, the acute toxicity of raw (untreated) and pretreated SADB on heterotrophic biomass was investigated employing a modified (COD-balanced), activated sludge inhibition test. The toxicity experiments demonstrated that the inhibitory effect of SADB towards sewage sludge could be completely eliminated when the effluent was pretreated with Fenton's reagent. (c) 2005 Elsevier Ltd. All rights reserved.