The invention relates to a method for controlling a process operating by means of the electrochemical potential, in which process complex ores and/or concentrates are treated in order to arrange the valuable components in the materials in a form appropriate for further processing and in which method electrodes made of material essentially similar to the materials to be treated in the process. According to the invention an impedance analysis in connection with the measurement of the electrochemical potential is carried out for the material to be treated in process in order to analyze the state of the solid surface and/or the state between the solid material and the intermediary material. The measured values are utilized in the adjustment of the process parameters. In order to carry out the impedance analysis voltage pulses are conducted into the material in at least one frequency and in at least one value of the electrochemical potential of the material.