Biological degradation of chloroguaiacols contained in sulphite pulp chlorine bleaching wastewater was studied in four parallel biological fluidised bed reactor systems-one single aerobic, one single anaerobic and two combined anaerobic-aerobic reactors. At low loading rates, trichloroguaiacols were removed nearly quantitatively. 4,5-Dichloroguaiacol was only partly removed. At high loading rates the anaerobic-aerobic recycle reactor removed individual guaiacols more than the other reactors. Only 4,5,6-trichloroguaiacol was removed best by the anaerobic-aerobic reactors in series. Even mixed culture biofilms adapted during several years of continuous operation did not satisfactorily remove these compounds. Synthetic wastewater, containing chlorinated guaiacols, treated with ozone produced formate and oxalate and quantitatively inorganic chloride. Combined ozonation-biotreatment in two reactors in series as well as in a recycle system allowed complete removal of all individual chlorinated guaiacols (<1 mu mol m(-3) remained). The efficiency of non-purgable organic carbon removal could be increased from less than or equal to 0.55 to about 4 mol carbon mol(-1) ozone by combination of ozonation with biotreatment. Simultaneously, the efficiency of removal of chlorinated guaiacols was increased by a factor of 10, which is essential for industrial application.