Experiments were conducted in an Anaerobic Batch Reactor (ABR) to assess the influence of pH, organic load and retention time on hydrogen production using pig manure as substrate. The study was conducted in two stages: the first stage focused on the effect of pH and the second stage assessed the effect of retention time and organic load on hydrogen production. The pH values investigated were 5.0, 5.5 and 6.0. The retention times were 12, 24 and 36 h with organic loading rates of 96.4, 48.2 and 32.1 kg VS/m(3)d, respectively. pH 5.5 had a maximum hydrogen concentration and production rate of 26.9% and 31.8 mL H-2/h, respectively. Meanwhile, the retention time and organic load of 12 h and 96.2 kg VS/m(3)d produced a maximum hydrogen concentration and production rate of 23.6% and 102.1 mL H-2/h, respectively. The hydrogen concentration obtained by ABR of pig manure is limited. Methanogenesis was inhibited as concluded from the methane concentrations being below 1% during all experiments except pH 6.0 and a retention time of 36 h. At pH 6.0, an inverse linear relationship between methane and hydrogen concentration was found. Finally, a modified Gompertz model was used to fit hydrogen production at retention time of 12 and 24 h. (C) 2012 Elsevier Ltd. All rights reserved.