In bioleaching processes using autotrophic bacteria, CO2 is the carbon source for the growth of the microorganisms and its availability is dependent on gas mass transfer in the bioreactor. In this study the demand in CO2 was investigated during bioleaching of several sulfidic materials (pyritic tailings, Cu concentrate, coal waste) in STR using the " BRGM-KCC " bacterial consortium. The results show (i) that Fe oxidation (and thus microbial activity) is delayed when air is injected without CO2-supplementation, and (ii) that CO2-supplementation improves leaching kinetics. The study proposes also a methodology to determine G/L transfer components and to asses CO2 limitations in the system. It shows that the microorganisms are not only sensitive to the transfer rate of CO2 from the gas to the liquid phase, but also to the availability of CO2 in solution.