The anaerobic mesophilic degradation of municipal waste has been studied in discontinuous lab-reactors with two different initial concentrations of OFMSW: R20, with 931.1 mg DOC/L (20% TS), and R30, with 1423.4 mg DOC/L (30% TS). The anaerobic digestion process was favoured when it was carried on material with a total solids content of 20% in comparison to a similar process with 30% TS. A higher level of organic matter, in terms of DOC and VFA, 18.18% and 8.09% respect, was removed in the system with the lower amount of solids. The kinetics parameters showed higher active biomass and a higher coefficient for the production of methane at the lower loading. The highest maximum specific growth rate for the microorganisms (mu(MAX)) in the reactors with 20% TS, a value of 0.192 d(-1), was achieved in comparison to 0.131 d(-1) in the reactors with 30% TS. The inverse of the F:M ratio was higher in the R20 system with a value of 0.0104 (cf. 0.0006 in R30) and, as a result, the R30 system required longer hydrolytic and acidogenic stages. X(vo)/Y(x/s) and the yield coefficient for product generation (alpha(P/S)) were higher, by around 53%, for the R20 digester. (C) 2010 Elsevier Ltd. All rights reserved.