Schottky barriers have been formed by vacuum evaporation of Cu on to CdS thin films. The behaviour of these samples has been investigated as a function of time and annealing by standard electrical methods: current-voltage analysis, capacitance-voltage analysis and analysis of the spectral dependence of the photoemission currents. The impurity profile deduced from the reverse differential capacitance shows evidence of copper diffusion occurring between 20 and 200 degrees C. An activation energy of 0.72 eV is found for the temperature dependence of the diffusion coefficient. The Cu-CdS interface has also been investigated by looking at the photovoltaic mechanisms in connection with the different heat treatments. Capacitance measurements performed under junction illumination have been used to obtain the true donor density after copper diffusion.