Water-based paint wastewater which is formed during the coating step of metal surfaces performed with various spray processes include binders, pigments and additives. Binders and other additives may also affect the toxicity of the paint depending on the specific characteristics of the paint. This study deals with Water-based paint wastewater, which is hard to treat. Fenton oxidation process was principally used to treat the wastewater. Treatability runs were carried out by using rust (oxidized iron) particles obtained as a waste of the metal rolling process, as the Fenton's catalyst. In order to change the variables such as dimensions of the column, packing material size, reactive dosage, pH value and reaction time, the experiments on the basis of packed columns and mixed reactors were carried out to increase the COD removal. COD eliminations with pH adjustment were about 20%. With the contribution of Fenton process in both column and mixed-reactor studies, the COD removal was improved by about 80%. (c) 2005 Elsevier Ltd. All rights reserved.