Mechanism of indirect and direct bacterial oxidation of sulphide mineral was discussed. Indirect oxidation occurred through aqueous ferrous solutions microbial catalysis to ferric ion oxidation and then direct oxidation of the sulfide by ferric ion. Experiments were conducted to distinguish between direct and indirect mechanism. The tests were based on the assumption that in the case of a direct mechanism a significant amount of O2 consumption was at the pyrite surface. It was found that all O2 consumption was not at the surface but in the solution which indicated that all bacterial substrate consumption was in the solution, which was a very strong argument in favour of an indirect mechanism.