This paper presents a mathematical model able to simulate under dynamic conditions the physical, chemical and biological processes prevailing in a biological sulfate reducing gas-lift reactor. The proposed model is based on differential mass balance equations for substrates, products and bacterial groups involved in a sulfate reduction process. Heterotrophic sulfate reducing bacteria (HSRB), autotrophic sulfate reducing bacteria (ASRB), homoacetogenic bacteria (HB), methanogenic archaea (MA) and acetate degraders (AD) are the microbial groups taken into account in the model. The model is also used to validate a steady-state design model previously proposed by Esposito et al. [1]. The proposed model is able to simulate the competition between the biological bacteria growing in the reactor, and predict the performance of a gas-lift reactor. The model includes two main parts: (1) a kinetic part including growth, metabolism and competition of SRB, HB, MA and AD in the system and (2) a mass-transfer part describing the thermodynamic concentration equilibria of gaseous components in the liquid and gas phase. The model has been validated using experimental data obtained by operating a laboratory-scale gas-lift reactor as described in Esposito et al. [2]. The model can be applied to simulate the sulfate reduction process in a gas-lift reactor for several purposes, such as the evaluation of the optimal process conditions in terms of COD:SO42- ratio, hydraulic retention time and gas input flow. In particular, model simulations reported in this paper show the model capability to predict the prevailing bacterial species and concentrations in the reactor as a function of the hydraulic retention time. (C) 2012 Elsevier Ltd. All rights reserved.