The intensive grinding of sphalerite or sphalerite-pyrite mixtures accelerates the extraction of zinc from sphalerite by the bacteria Thiobacillus ferrooxidans and as a consequence shorten the process of bacterial leaching. This grinding affects the solid-state properties of the sulphides. It increases not only the specific surface but also the defects in sphalerite that have a positive influence on the progress of bacterial leaching. Such intensive grinding modifies in complement the electrochemical properties by increasing the anodic behaviour of sphalerite in a sphalerite-pyrite mixture. The extent of the changes in leaching is influenced by agglomeration effects of small particles obtained by intensive grinding as well as by the presure of pyrite that increase the galvanic effect and the surface properties.