Abstract: The Role of biosorption in the removal of AOX in the secondary treatment of pulp CEH bleaching effluents was studied. The result shows that the biosorption behavior can be characterized by Langmuir isotherm, the sorbed AOX from aerobic sludge is greater than that from anaerobic sludge,and the sorbed AOX of low molecular weight is greater than that of high molecular weight. The desorption experiment proved that biosorption is an irreversible process; the sorbed AOX by dead biomass is not significantly different from that by live biomass and the sorption enthalpy was 0.37 and 0.38 kJ/mol respectively for aerobic and anaerobic sludge,which confirms that biosorption is only a physical sorption bound by van der waals forces.