This study elucidated the mechanism and kinetics of ozone decolorization and dechlorination of composite wastewaters obtained from various pulp mill effluents by means of ultrafiltration and freeze drying. These raw samples had similar initial true color but varied color intensity (C.I.) and biodegradability. Both raw and ozone-treated samples were separated into four components to assess the effects of ozonation on individual components. Analytical HPSEC was employed to determine the changes induced by ozonation in molecular weight distribution of lignins and their derivatives.