Hydrogen production (HP) from the organic fraction of municipal solid waste (OFMSW) under thermophilic acidogenic conditions was studied. The effect of nine different organic loading rates (OLRs) (from 9 to 220 g TVS/l/d) and hydraulic retention times (HRTs) (from 10 d to 0.25 d) was investigated. Normally, butyrate was the main acid product. The biogas produced was methane- and sulfide-free at all tested OLR. Increasing the OLR resulted in an increase in both the quantity and quality of hydrogen production, except at the maximum OLR tested (220 g TVS/l/d). The maximum hydrogen content was 57% (v/v) at an OLR of 110 g TVS/l/d (HRT = 0.5 d). HP was in the range of 0.1-5.71 H-2/l/d. The results have clearly shown that the increase in OLR was directly correlated with HP and microbial activity. The bacterial concentration inside the reactor is strongly influenced by the content of microorganisms in the OFMSW. (C) 2012 Elsevier Ltd. All rights reserved.