The effects of chlorine dioxide substitution for elemental chlorine in the chlorination stage, aimed at the minimization of chlorinated organics and subsequent oxygen reinforcement in the proceeding extraction stage on the characteristics of bleaching effluents have been examined. The characteristics of the effluents were assessed in terms of molecular weight distribution by gel filtration, so that alternatives for the treatment of these effluents can be proposed. The analysis of chlorination and extraction stage effluents with different chlorine dioxide substitution levels and varying available chlorine charges by using a Sephadex G-50 column have indicated that indeed a marked change in the molecular weight distribution of chlorinated organic compounds have occurred. The molecular size of chlorinated organic compounds in the chlorination stage has increased with the chlorine dioxide substitution, while that in the extraction stage has decreased when compared with the molecular size of these compounds in original bleaching effluents with only elemental chlorine used in the chlorination stage and followed by extraction stage. Molecular size of the chlorinated organic compounds in the chlorination stage effluents was not seriously affected by the chlorine dioxide substitution level; while ii was increased greatly by the increased total chlorine dosage. In the-extraction stage effluents, chlorine dioxide substitution level has not appeared to be an important parameter influencing the molecular size distribution. (C) 1997 IAWQ. Published by Elsevier Science Ltd.