Simulation and modeling of continuous H-2 production by Enterobacter cloacae IIT-BT 08 using different bioreactor configurations have been carried out and a suitable kinetic model for the substrate degradation has been proposed, assuming ideal plug flow, no axial dispersion, steady state and first-order substrate degradation kinetics. The degradation kinetics was investigated as a function of flow rates considering the gas hold-up in all the bioreactors. The external film diffusion model of the type J(D) = KNRe-(1-n) is proposed for a tubular bioreactor and JD is found to be constant in the present system. The same model is applied to both tapered and rhomboidal bioreactors and was found to give reasonably good fit with experimental results with respect to substrate conversion. Comparison of the external mass transfer coefficient k(1) with the observed first-order reaction rate constant k(p) predicted by the model shows that the effect of external film diffusion on the observed substrate degradation rate decreases with increasing flow rates. For the present system, both substrate degradation and mass transfer steps are the limiting factor for the observed substrate degradation rate, particularly at higher dilution rates.