Hydrogen (H-2) is a crucial electron donor for many processes in the environment including nitrate-, sulfate-and, iron-reduction, homoacetogenesis, and methanogenesis, and is a major determinant of microbial competition and metabolic pathways in groundwater, sediments, and soils. Despite the importance of H-2 for many microbial processes in the environment, the total H-2 consuming capacity (or H-2 demand) of soils is generally unknown. Using soil microcosms with added H-2, the aims of this study were 1) to measure the H-2 demand of geochemically diverse soils and 2) to define the processes leading to this demand. Study results documented a large range of H(2 )demand in soil (0.034-1.2 millielectron equivalents H(2 )g(-1) soil). The measured H-2 demand greatly exceeded the theoretical demand predicted based on measured concentrations of common electron acceptors initially present in a library of 15 soils. While methanogenesis accounted for the largest fraction of H(2 )demand, humic acid reduction and acetogenesis were also significant contributing H-2-consuming processes. Much of the H-2 demand could be attributed to CO(2 )produced during incubation from fermentation and/or acetoclastic methanogenesis.The soil initial total organic carbon showed the strongest correlation to H-2 demand. Besides external additions, H(2 )was likely generated or cycled in the microcosms. Apart from fermentative H-2 production, carboxylate elongation to produce C4-C7 fatty acids may have accounted for additional H(2 )production in these soils. Many of these processes, especially the organic carbon contribution is underestimated in microbial models for H-2 consumption in natural soil ecosystems or during bioremediation of contaminants in soils.