The purpose of this paper was to study the evolution of microbial community and its relation to the hydrogen production (HP) steps in thermophilic-dry dark fermentation from real organic fraction of municipal solid waste (OFMSW). Nine organic loading rates (OLRs) (from 9 to 220 g TVS/1/d) were investigated. Population dynamics study showed that increasing OLR (between 9 and 110 g TVS/1/d) resulted in an increase in the relations between Eubacteria:Archaea and hydrolytic-acidogenic bacteria (HABs):acetogens. This was strongly influenced by the microbial content of the OFMSW. The presence of acetogens and Archaea was due to contribution of these microorganisms in the substrate (the biogas produced was methane-free). The maximum value of hydrolysis (63 +/- 7%) was observed at 110 g TVS/I/d OLR according to maximum HP and HAB activity. The highest average values of acidification yields (57-60%) were achieved for OLR between 28 and 43 g TVS/1/d. (C) 2013 Elsevier Ltd. All rights reserved.