Hydrogen is an energy source that can be produced by Clostridium sporogenes microorganism. In the present work, modeling of dark fermentation using Clostridium beijerinckii and dextrose as substrate was performed to evaluate how the gases and liquid by-products affect the biological process. A mathematical model was developed according to ADM1. The developed model takes into account biochemical reactions, physicochemical equilibrium as well as mass transfer processes during dark fermentation. Findings revealed that Clostridium beijerinckii reached a yield as high as 3.58 mol of H-2 /mol of dextrose and generates by-products in the aqueous phase that may either be used as raw materials in a chemical process. Clostridium beijerinckii is very sensitive to acid media (pH < 5.0) and shows a low rate of biohydrogen production (even the absence of metabolic activity) at pH lower than 4.5. The developed model is able to predict (R-2 > 0.95) dextrose consumption profile, cumulative biohydrogen production and the maximum concentrations of liquid by-products. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.