The molecular weight distribution (MWD) of chlorinated compounds of bleached Kraft pulp mill effluent was investigated by aqueous gel permeation chromatography (GPC) and ultrafiltration. The effluent was fractionated by ultrafiltration, using different cut-off membranes (MW 50 000, 20 000, 10 000 and 8000). The retentate and permeate of each ultrafiltration was analyzed by GPC. The results showed that the molecular weight distribution for all samples ranged from 200 to 550 Da. Two effects were discussed: associative interactions between the compounds to form high molecular associated complexes (aggregates); and non-size-exclusion effects, like ion exclusion. Aggregates formation was confirmed by ultrafiltration experiments, using a 50 000 cut-off membrane.