Simultaneous measurements of pH and redox potential using a platinum electrode as well as poten- tial of copper sulphide electrodes during non-oxidative leaching of Lubin shale middlings with sulfuric acid have been applied to determine leaching kinetics and to evaluate its electrochemical parameters as well as to assess the changes taking place at the mineral/solution interface during leaching process. Significant decrease in both redox potential of the solution (from +350 mV to +50 mV, Ag,AgCl) and potential of mineral electrodes were observed with increasing pH during carbonate decomposition with sulfuric acid. It is therefore concluded that the low value of red-ox potential is a parameter preventing oxidation of copper sulphide minerals present in leached material and prevents the transition of Cu and Fe to the solution. It was found that the non-oxidative leaching is a rapid and selective process, resulting in decomposition of calcium and magnesium carbonates and in saturation of the leaching slurry with carbon dioxide. It was recognized that with the increasing degree of carbonate decomposition from 50 to 100 % metals-bearing minerals liberation has remarkably improved, which beneficially facilitates and intensifies further upgrading or leaching processes.