Anaerobic processes are often prescribed as the first treatment step for pulp and paper mill wastewaters with high COD concentration, in order to meet the corresponding effluent requirements. These type of wastewaters contain an inert COD fraction resistant to aerobic degradation. This inert fraction of prime significance for compliance monitoring is meaningful when defined in accordance with the treatment process applied. In this paper the application of a new method proposed for the assessment of inert COD fractions under aerobic conditions, is tested on a pulp and paper mill wastewater sample, under anaerobic conditions and the results have been reported and evaluated. (C) 1998 Elsevier Science Ltd. All rights reserved.