Aims: To characterize cellulolytic, hydrogen-producing clostridia on a comparable basis. Methods and Results: H-2 production from cellulose by six mesophilic clostridia was characterized in standardized batch experiments using MN301 cellulose, Avicel and cellobiose. Daily H-2 production, substrate degradation, biomass production and the end-point distribution of soluble fermentation products varied with species and substrates. All species produced a significant amount of H-2 from cellobiose, with Clostridium acetobutylicum achieving the highest H-2 yield of 2.3 mol H-2 mol(-1) hexose, but it did not degrade cellulose. Clostridium cellulolyticum and Clostridium populeti catalysed the highest H-2 production from cellulose, with yields of 1.7 and 1.6 mol H-2 mol(-1) hexose from MN301 and 1.6 and 1.4 mol H-2 mol(-1) hexose from Avicel, respectively. These species also achieved 25-100% higher H-2 production rates from cellulose than the other species. Conclusons: These lytic, hydrogen-producing clostridia varied in H-2 production, with Cl. cellulolyticum and Cl. populeti achieving the highest H-2 yields and cellulose degradation. Significance and the Impact of the Study: The fermentation of cellulosic materials presents a means of H-2 production from renewable resources. This standardized comparison provides a quantitative baseline for improving H-2 production from cellulose through medium and process optimization and metabolic engineering.