Auger depth profiling combined with X-ray photoemission and Rutherford backscattering has been used to study diffusion processes in Cu films exposed to C12 at 25°C under a large range of exposure conditions. The diffusion depths and coefficients as a function of the gas pressure and the exposure time are determined and analyzed. It is found that the pressure and the time dependences of the diffusion properties are rather complex and there is a drastic change in the diffusion behavior in the pressure range between 10 -3 and 10-2 Torr. Below 10-3 Torr, the average Cl concentrations in the surface regions are small and the diffusion depths are shallow. Above 10-2 Torr, the surface Cl concentrations are high and rather thick surface layers containing constant Cl concentrations extending several thousand Å or more into the bulk of the metal are found. The diffusion processes clearly affect the surface chlorination reaction of copper. These and other effects including the depth profiling technique are discussed. © 1986.