Filtration and ultrafiltration with a size range of 2-1600nm were used to evaluate the effect of ozonation on the particle size distribution-based chemical oxygen demand (COD) and color profiles of textile wastewater before and after biological treatment. Ozonation induced a net effect of 9% COD reduction in the influent and 15% in the effluent. However, a more in-depth evaluation based on particle size distribution and mass balance for the influent revealed different mechanisms of ozonation, which were interpreted as total oxidation in the soluble range, replenishment of soluble COD through solubilization of organics into simpler compounds and polymerization towards the upper size range (> 220 nm). For the biological treatment effluent, the greatest effect of ozonation was in the lower particle size range (< 8 nm). Ozone was very effective for color removal, giving 80-93% optical density reductions in the influent and 96-99% in the effluent, depending on the excitation wavelength selected. Ozonation of the influent removed practically all color fractions, except in the particulate range. In the effluent, the particulate fraction was removed by biological treatment and settling and consequently the remaining color were almost entirely removed by ozonation. (c) 2005 Society of Chemical Industry.