Composition of the Organic Fraction of Municipal Solid Waste (OFMSW) in organic compounds and inorganic ions is highly variable and might impact the microbial activity in dark fermentation processes. In this study, the effect of the total amount of inorganic ions on fermentative hydrogen production was investigated. Batch experiments were carried out at pH 6 and under a temperature of 37 degrees C. A freshly reconstituted organic fraction of municipal solid waste (OFMSW) was used as model substrate. At low concentrations in ammonium or chloride ions (2.9-5.1 g L-1, respectively), the hydrogen yield reached a maximum of 40.8 0.5. mLH(2).gVS(-1) and 25.1 5.6 mLH(2).gVS(-1). In contrast, at high total ionic concentrations of ammonium and chloride (11.1-35.5 g L-1 respectively), a strong inhibition of the fermentative microbial activity and more particularly hydrogen production, was observed. When considering the ionic strength of each ion, the effects of ammonia, chloride or a mixture of different ions (Na+, K-+,K- H+, NHZ, Mn-2, NH4, me(+), Cl-, poi-, Br-, 1-, Sail showed very similar inhibitory trends regardless the type of ion or the composition of the ionic mixture. A threshold inhibitory value of the ionic strength was estimated at 0.75 0.13 M with a substantial impact on the fermentative activity from 0.81 0.12 M, with hydrogen yields of 18.1 3.3 and 6.2 4.1 mLH(2).gVS(-1), respectively. Microbial community composition was also significantly impacted with a specific decrease in relative abundance of hydrogen -producing bacteria from the genus Clostridium sp. at high ionic strength.