A solid electrochemical cell copper-metal/electrolyte/mineral was studied by impedance spectroscopy between 10(5) and 2 x 10(-3) Hz in the temperature range 120-165 degrees C. The mineral was a mixture of hexagonal chalcocite and high digenite of natural origin with a composition Cu1.88S. The electrolyte was RbCu4Cl5. The impedance spectra at equilibrium and low cathodic over-voltages showed two distinct regions. At high frequencies, the general shape of the diagram was a depleted are of a circle. At low frequencies, a diffusion branch was observed but not when the working electrode was replaced by copper metal. The results were interpreted in terms of copper insertion into the sulfide. The real contact area was determined and the impedance parameters were calculated and analyzed. At 145 degrees C the chemical diffusion coefficient of copper vacancies calculated from impedance data is 2.0 x 10(-7) cm(2) s(-1).