Physical and chemical characteristics of six types of pyrite isolated from ores and coals as well as the mechanism of their oxidation by Thiobacillus ferrooxidans and Sulfobacillus thermosulfidooxidans were investigated. The effect of monocrystallinity, orientation of crystals, and-microaggregation on the oxidation degree of polish rock section was clearly revealed. The investigation of the surface of powdered pyrite particles showed the ratio S2-/Fe2+ of pyrite to be equal to 2.7. This indicates that the surface of the pyrite particles was deficient in metal or redundant in sulfur. This ratio holds in the coarse of pyrite oxidation by T. ferrooxidans, indicating that Fe2+ and S2- are oxidized simultaneously. In the presence of S. thermosulfidooxidans the ratio S2-/Fe2+ is reduced to 1.0 for some types of pyrite indicating preferential oxidation of S2-. The kinetics and mechanism of bacterial oxidation of pyrites are discussed.