Combined effluents of a Kraft pulp mill using yearly plants were treated in a lab-scale activated sludge system. Average TOC and AOX removal efficiencies were 83% and 21% respectively. The effect of the operating solids retention time (SRT) on AOX removal was investigated and it was found that highest AOX removal (30%) occurs at longer SRT's. Mass balance on the system revealed that the principal AOX removal mechanism was metabolization at long SRT. About 90% of the AOX removed was metabolized. As SRT was lowered both AOX removal efficiency decreased and AOX removal mechanism changed to removal by wastage of the AOX adsorbed onto the daily wasted biomass. The effect of influent AOX concentration and additional carbon source on microbial activity was investigated by performing batch tests. The presence of additional COD increased the activity, however, the effect of influent AOX concentration was not clear.