This paper reports the use of an anaerobic baffled reactor (ABR) and effect of different pH, temperatures, hydraulic retention times and organic loading rates on continuous anaerobic digestion of black liquor from pulp and paper mills. A maximum COD reduction of about 60% at an organic loading rate (OLR) of 5 kg m(-3) d(-1) at hydraulic retention time of 2 d, pH 8.0 and 35 degrees C was recorded. The OLR above 6 kg m(-3) d(-1) were toxic and destabilised the reactor system.