Two samples of chalcocite with compositions Cu1.9984S and Cu-1.9962 were studied. They were first characterised by electrochemical, electrical and X-ray diffraction measurements. For the monoclinic gamma-phase, the variations with temperature and composition of the chemical diffusion coefficient of copper in these mixed conductors were investigated using two different methods: measurements of the time dependence of the resistance change of the samples after application of a small dc voltage at an ion blocking microelectrode (between room temperature and approximate to 100 degrees C) and electrochemical impedance spectroscopy (EIS) between room temperature and 58 degrees C. The chalcocite hexagonal beta-phase (>100 degrees C) was studied using a microelectrode, showing a small decrease of (D) over tilde(Cu) at the phase change.