This paper describes the results of copper coupons exposed to a class III mixed flowing gas environment (MFG) following the guidelines given by the Battelle Laboratory and the International Electrotechnical Commission for environmental testing. Corrosion products were studied in detail using scanning electron microscope, energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), focused ion beam (FIB), secondary ion mass spectroscopy (SIMS), and transmission electron microscope. The weight gain measured after each exposure was compared with the weight gain calculated from the cathodic reduction of the corrosion layers and cross sectioning using an FIB. The result shows a relatively good correlation between the measured and the calculated experimental values of weight gain. As expected, within the first week, the different corrosion layers thickened until they formed a thick layer that became the determining step for further growth. After several days of exposure the Cu coupons developed a complex multilayered structure consisting of cuprous oxide (Cu2S), cupric oxide (CuO), copper sulfide (Cu2S), covellite (CuS), and evidence of antlerite (3CuO SO3 2H(2)O). No Cl-containing corrosion products were identified using XRD. However, EDS and SIMS analysis showed that Cl was distributed throughout the corrosion products, indicating that although Cl is inside the corrosion products, it is not part of the crystalline structure. Also, this suggests that Cl plays an important role in accelerating the corrosion of Cu during exposure to the MFG class III test. (c) 2008 The Electrochemical Society.