Auger electron spectroscopy in conjunction with argon⁺ sputtering has been used to investigate four samples of copper that have been patinated by exposure to the atmosphere for periods ranging from 1 to 100 years. The elemental depth profiles were qualitatively similar for all samples. The patinas were covered with a thin layer of carbon-containing species, and consisted primarily of copper, sulfur, chlorine, and oxygen. The sulfur is present primarily near the surface of the patina, and the chlorine is distributed throughout the patina. In two samples, 44 and 100 years old, significant amounts of chlorine were found in the portion of the patina nearest the bulk copper. A simple mass balance shows that the surface of the patina (beneath the carbon-containing layer) is composed primarily of the mineral brochantite, Cu₄(SO₄)(OH)₆, as expected thermodynamically. A thermodynamic analysis also shows that atacamite, Cu₂Cl(OH)₃, the principal chlorine containing mineral found in the patinas, can be stable with respect to cuprite, Cu₂O, particularly in the immediate vicinity of unoxidized copper. -- AATA