The removal of chlorinated organic compounds from bleached kraft pulp mill effluent in an operating biological treatment system, consisting of an aerated lagoon and settling pond, was studied. The system performed extremely well for reduction of BOD. Preliminary results indicate that the overall removal of AOX was in the range of 15 to 47% across the entire system with the majority being removed in the aerated lagoon. Removal of low molecular weight (<1000 dalton) AOX was greater at 43–50%. About one third (range 30–36%) of the TOC was removed and the ratio of AOX to TOC was relatively constant at about 0.124 (g AOX/g TOC) throughout the lagoon suggesting that the mechanisms of removal were the same for both. The amount of chlorine bound to the organic matter in the sludge and in the interstitial water was found to be relatively constant throughout the lagoon. Significantly less AOX was found in the interstitial water than in the overlying water.