Batch anaerobic reactor experiments were set up in which the degradation of the primary substrates cellulose and glucose was assayed when dosed with a concentration range from 1 to 20 g l(-1) of a synthetic mixture of volatile fatty acids (VFA). Biogas production from the VFA mix and from reactors without VFA additions was used as a baseline control against which the results were compared and interpreted. The l-1 mesophilic (35 degrees C) reactors were seeded with an actively digesting sludge of sewage origin and monitored for biogas production, gas composition, volatile fatty acid concentration, glucose content and the cellulolytic enzymes carboxymethylcellulase and avicelase. Cellulose reduction was measured from initial and final samples of the reaction mix in each case. VFA caused inhibition of the cellulolytic activity at concentrations >= 2 g l(-1), and therefore of the rate of cellulose hydrolysis. The fermentation of glucose was slightly inhibited at VFA concentrations above 4 g l(-1). The inhibitory effect on the production of biogas and also on the methane to carbon dioxide ratio was evident above 6 g l(-1) VFA in the initial mixture when used as the sole substrate. In combination with paper as primary substrate, biogas production due to the paper was more than halved above 1 g 1(-1) initial VIA, indicating inhibition of the hydrolysis process. Where glucose was the primary substrate biogas production was more than halved above 8 g which indicated that the fermentation was less sensitive to inhibition caused by VFA. (c) 2005 Elsevier Ltd. All rights reserved.