In order to further our understanding of the removal of adsorbable organic halogen (AOX) from bleached kraft pulp mill effluent in an operating lagoon, controlled batch studies (at 22°C and 30°C) were conducted under aerobic and anaerobic conditions. The removal of AOX ranged from 17 to 70% and the presence of sludge significantly increased the removal of both AOX and total organic carbon (TOC). There was no further significant removal of AOX beyond two days for the aerobic experiments and beyond 10 days for the anaerobic experiments. The removal of AOX under anaerobic conditions was not significantly different from those under aerobic conditions and increasing the temperature caused a significant increase in the removal of AOX. There was no significant difference in the overall removal or patterns of removal between AOX and TOC. A mass balance showed that the sludge represented the major fraction of organically bound chlorine in the mixed liquor.