Ozone is a strong oxidant, which can oxidize both biodegradable and non-biodegradable organics. The main objective of this study was to use iron oxide as a heterogeneous catalyst to enhance the ozone oxidation process. The wastewater used in this study was distillery wastewater, which was diluted 20 times before use. The diluted distillery wastewater was fed continuously in a downflow direction in an ozonation column. The iron oxide catalyst was coated on 10.3 mm diameter alumina balls (5.5 m(2)/g Specific surface area) by using Fe(NO3)(3) as a precursor. The prepared catalyst was in the form of ferric oxide, and its loading was 0.07%. From the experimental results of both with and without the iron oxide catalyst, an increase in hydraulic retention time resulted in an increase in the treatment efficiencies of both chemical oxygen demand (COD) and color reduction, since the residence time of ozone increased. When the ozone mass flow rate increased, both COD and color reduction increased, resulting from an increase in the hydroxyl radical available in the system. The ozonation system with the iron oxide catalyst gave the highest efficiency in both COD and color removals because the hydroxyl free radical generated from the catalyst is more reactive than the ozone molecule itself. (C) 2007 Elsevier B.V. All rights reserved.