In this study, grass silage was used both as a source of bacteria and as a substrate for dark fermentative hydrogen production. Silage is produced by lactic acid fermentation controlled by end point pH (<4.0). In this study, the fermentation of silage was successfully continued and directed to hydrogen production by neutralizing the pH. Highest hydrogen yield of 37.8 +/- 5.8 mL H-2/g silage was obtained at 25 g/L of silage. The main soluble metabolites were acetate and butyrate with the final concentrations of 1.5 +/- 0.2 and 0.5 +/- 0.0 g/L, respectively. Bacteria present (at 25 g silage/L) included Ruminobacillus xylanolyticum, Acetanaerobacterium elongatum and Clostridium populeti and were involved in silage fermentation to hydrogen. In summary, this work demonstrates that grass silage becomes amenable to hydrogen fermentation by indigenous silage bacteria through pH neutralization. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.