A novel biological sulfite/sulfate reduction process has been evaluated using a rate-based mathematical model. The principle of this process is that sulfate reducing bacteria reduce sulfate or sulfite to sulfide using H-2 and CO2 as energy and carbon source under anaerobic conditions. The H2S gas resulting from the biological sulfate reduction is simultaneously stripped off and, in a separate stage, absorbed and oxidized to elemental sulfur. In this novel process, the biological sulfate/sulfite reduction reaction is accompanied by absorption of H-2 and CO2, chemical reactions in the liquid phase and the subsequent desorption of the volatile reaction product, H2S. As a result of the complicated nature of this process, accurate simulation of the system is essential for the precise design, parameter estimation and control of the process. The proposed model adopts the film theory and includes both instantaneous equilibrium reactions and reactions with finite rates. Mass transfer resistance in the gas phase is also taken into account. The rate based process model was implemented in a packed-bed bioreactor model to obtain reactor dimensions as a function of operational variables. A sensitivity analysis was conducted on the mathematical model to define the key parameters of the process design and performance, and to establish the optimal reactor operational conditions. (c) 2005 American Institute of Chemical Engineers.