This study presents a mathematical model for predicting volatile fatty acid (VFA) production by dark fermentation of complex particulate substrates of biomass origin. This model is based on the premise that degradable components of such substrates are first hydrolyzed into soluble monomers, which are then fermented to liquid and gaseous end products. The model considered cellulose and hemicellulose as the degradable components of typical biomass-based organic substrates; and volatile fatty acids, hydrogen, and carbon dioxide as the end products. Hydrolysis of the particulate forms of the components is modeled by a surface-limiting rate equation, and the formation of end products is modeled following the guidelines of the anaerobic digestion model (ADM1). The model was calibrated and validated using experimental COD and VFA data from six batch reactors where, cattle manure was tested as a representative biomass-based substrate. The model predictions agreed well with the temporal trend in the experimental data, with overall r(2) of 0.85 for COD, 0.82 for acetic acid, 0.72 for butyric acid, and 0.42 for propionic acid. (C) 2010 Elsevier Ltd. All rights reserved.