This paper describes the results of copper coupons exposed to a class III mixed flowing gas environment ͑MFG͒ 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 ͑EDS͒, X-ray diffraction ͑XRD͒, focused ion beam ͑FIB͒, secondary ion mass spectroscopy ͑SIMS͒, 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 ͑Cu 2 S͒, cupric oxide ͑CuO͒, copper sulfide ͑Cu 2 S͒, covellite ͑CuS͒, and evidence of antlerite ͑3CuO SO 3 2H 2 O͒. 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. In the early 1980s, with the discovery of significant printed wiring board ͑PWB͒ and component failure modes ͑mainly due to corrosion͒, a number of firms and laboratories set out to develop accelerated corrosion test methods with a known acceleration factor. The aim of such efforts was to shrink years of service into days of testing, and prove that the field failure modes would be replicated during the tests. The PWB and its components were exposed to different levels of a mixture of gases, temperature, and relative humidity, which would simulate the environment during operating conditions. IBM, AT&T, and Battelle Laboratories participated in this effort.1 The result of this work was the development of a mixed flowing gas ͑MFG͒ test, which is primarily a laboratory test in which the temperature, relative humidity, and concentration of gaseous pollutants are carefully defined, monitored, and controlled.