The lack of adequate gas mass transfer is a potential rate limiting step in many bacterial leaching processes. Oxygen can become a limiting factor because of its low solubility compared to the high demand induced by sulfide oxidation. One way of increasing the solubility of oxygen in water or in media solution is by increasing the driving force, i.e. raising the oxygen partial pressure in the gas stream supplied to the leach pulp. The use of oxygen is a well-known practice in high-temperature bioleaching reactors (above 70°C) whereas air is usually preferred in medium and low-temperature operations, mainly for practical and economic reasons in classical CSTR bioleaching condition.Another reason associated with not using enriched oxygen gasis to avoid too high dissolved oxygen concentrations which could impact negatively the bacterial activity. The purpose of this study was to investigate the use of oxygen-enriched gas in bioleaching reactors at 40°C in order to improve the gas transfer in the system when operating at high solid load (20% and more). Bioleaching experiments were performed on a sulfide-rich tailing waste (pyrite 60%) using the " BRGM-KCC " bacterial consortia. The reactor used for the tests was designed on the basis of a new bioleaching reactor concept developed by Air Liquide, Milton Roy Mixing and the BRGM which uses a floating agitator to inject gases, and to mix and suspend solids in the bioleaching solution.Two types of tests were carried out: with air injection and with oxygen enriched gas mix injection.The aim of this work was to confirm the capacity of bacteria to grow and to dissolve pyrite in this type of bioreactors in oxygen-rich atmosphere, and to compare sulfides dissolution rates during the two types of bioleaching tests. The profitability of this new concept of bioreactor will relyon the optimisation of oxygen transfer. The results obtained show that the use of oxygen enriched gas mix does not negatively impact bioleaching performances compared to air injection. High metal extraction yields were achieved (above 80%). No deleterious effect due to oxygen use was observed on the bacteria despite the high level of dissolved oxygen reached in oxygen tests (up to 14 mg.L-1). This result is very encouraging in the development of the new type of bioreactorwhere limited gas flows can be combined with high oxygen transfer rates and efficiency. KEYWORDS