Kraft pulp mill foul condensates were treated in a 4-L membrane bioreactor, which was fed continuously with a chemical oxygen demand (COD) load of 5.12 k- COD/m(3)/day, and a hydraulic retention time (HRT) of 19 h. During a period of 60 days, three temperatures were tested 35, 45 and 55 C. Very high COD, total reduced sulphur (TRS) and methanol removals were obtained in the mesophilic temperature range. As the temperature increased, a decrease in efficiency was observed, although it could still considered to be high. A decrease in respiratory activity from 19.98 mg O-2/g TSS/h at 35 degreesC to 12.87 mg O-2/g total suspended solids (TSS)/h obtained at 55 degreesC, followed by a decrease in the TSS concentration in the bioreactor, suggested that the biomass is thermotolerant. The results of this research showed that the treatment of foul condensates from kraft pulp mills at high temperatures using a membrane bioreactor was shown to be technically feasible and it has good potential for industrial application. (C) 2004 Elsevier Ltd. All rights reserved.