Reflection–absorption infrared spectroscopy, Auger electron spectroscopy (AES), temperature programmed desorption, and reactive scattering were used to investigate the adsorption and desorption behavior of PEt3on copper and chlorinated copper surfaces under ultrahigh vacuum conditions. No reaction was observed between PEt3and clean Cu(100) or between PEt3and ac(2×2)–Cl overlayer. At temperatures above 320 K, PEt3reacted rapidly with a heavily chlorinated copper surface (10 000 L exposure of Cl2at 300 K). Subsequent examination of the substrate by AES showed removal of chlorine and presumably copper. The remaining chlorine corresponded roughly to 0.5 monolayer coverage. This is consistent with reactive scattering, infrared and AES experiments carried out on thin chlorinated layers (8 L Cl2exposure), where essentially no reaction of PEt3with the surface was observed. The reaction between PEt3and a heavily chlorinated Cu(100) surface yields both ClCu(PEt3)2and Cl2PEt3. These data are consistent with the etching of copper under ultrahigh vacuum conditions at low temperatures by reaction with Cl2and PEt3to form CuCl(PEt3)2.