In the presented study the co-fermentation, fermentation of sewage sludge (SS) and fermentation of organic fraction of municipal solid waste (OFMSW) were compared. The goal of experiments was determination of the processes carbon balance and proposal of the simple kinetic model of anaerobic digestion. Three batch experiments with above mentioned feedstocks were conducted in large scale laboratory reactor of working volume of 40 dm(3). The cumulative biogas production for sewage sludge (180.59 dm(3)) was lower than that for co-fermentation (232 dm(3)) or OFMSW (228.34 dm(3)). During fermentation of the OFMSW an accumulation of volatile fatty acids (VFA) caused pH decrease and strongly inhibited gas production. The addition of co-substrate improved buffering capacity of fermentation broth. The suggested mathematical descriptions of carbon division in anaerobic digestion processes gave the satisfying conformity with the experimental values. (C) 2007 Elsevier Ltd. All rights reserved.