The results of microbial oxidation under laboratory conditions of mixed pyrite mill tailings from power industry by Acidithiobacillusferrooxidans bacteria have been presented in the paper. The analysis of the dynamics of Fe(II)/Fe(III) concentration changes, oxidizing-reducing potential Eh and pH as well as phase analysis revealed that despite a significant activity of microorganisms in microbial oxidation process, the level of iron releasing from wastes is limited by the process of precipitation of low-soluble iron(III) compounds.