A two-phase fermentation process for the treatment of waste, intended for the recovery of hydrogen for energy use, was investigated in its initial fermentation phase. Hydrogen production was obtained from a mixed culture based on an active mesophilic inoculum without any selective treatment being applied. The liquid stream generated by the hydrogen fermentation process was stabilized in the following, methanogenic, phase for the recovery of methane and further breaking down of the waste stream. The whole process was carried out at a temperature in the mesophilic range (34 degrees C). The substrate used was an unsterilized mixture of the organic fraction of municipal solid wastes (OFMSW) and slaughterhouse waste from a poultry-processing plant. The hydrogen-producing phase was capable of stable performance under the hydraulic retention times (HRTs) evaluated (3 and 5 days). No methane was detected in the first phase at any point during the whole period of the experiment and the hydrogen yield showed no symptoms of declining as time elapsed. The amount of hydrogen obtained from the fermentation process was in the range of 52.5-71.3 N-1 kg(-1) VSrem. (c) 2006 Elsevier B.V. All rights reserved.