Batch biodegradation kinetic studies were conducted with various bleached kraft pulp mill effluents containing organically bound chlorine measured as adsorbable organic halogen (AOX) (6.0 - 35 mg CI/L). Biomass, generated in both a continuous bench scale reactor and a mill scale activated sludge system, at various concentrations (0.4 - 3.1 g VSS/L) was added to various effluents (about 70% ClO2 substitution). AOX removal kinetics were best described by a simple recalcitrant first order model : - (dS)/dt = k(s/1)X(S - S-infinity), where S and S-infinity are AOX concentrations (mg C1/L) at time t and t(infinity), respectively, k(s,1) is the AOX removal rate constant (L/g VSS.h) and X is the biomass concentration (g VSS/L). The model indicated a constant non-biodegradable fraction of AOX (0.4 mg of AOX per mg of influent AOX) and was able to predict the effect of dilution on AOX removal with a single value of k(s,1) (0.13 L/g VSS.h). The degradation constants (0.16 L/g VSS.h) in the absence of effluent from the neutral sewer (i.e. for a mixture of acidic and alkaline effluent only) were significantly higher than those for whole mill effluent.