The chlorination behavior on Cu surfaces has been investigated using the quartz-crystal-micro-balance technique, X-ray photoemission. Auger spectroscopy, X-ray diffraction and Rutherford backscattering in the exposure range from 10 to 1010 L. The surface reaction rates and reaction products are determined and analyzed based on the spectroscopic data, in particular, the XPS and Auger measurements. Upon C12 exposure, a surface layer with an average stoichiometry CuClx is formed and the value of x can vary continuously from 0 to almost 2 as a function of the gas pressure and the exposure time. The reaction does not proceed as a simple layer-by-layer growth of stoichiometric CuCl and the surface composition is greatly affected by the rates of bulk diffusion. Under relatively low exposure conditions with x <1, the surface layer is found to consist of Cu and CuCl while at very high exposures with x > 1, coexistence of CuCl and CuCl2 is detected. The Cu-Cl2 reaction is also compared with other metal systems. It is suggested that a reaction mechanism similar to the Cabrera-Mott theory of oxidation is responsible for the observed Cu chlorination reaction. © 1986.