The objective of the work reported here was to characterize the tarnish products formed on copper during the early stages of exposure to moist air with trace levels of pollutant gas, in particular, sulfur dioxide and sulfur dioxide/nitrogen dioxide. A mixed-flowing-gas chamber was used to control part per billion (ppb) gas concentrations, relative humidity, temperature, and flow conditions. Two analytical techniques, x-ray photoelectron spectroscopy and coulometric reduction (chronopotentiometry), were utilized to study the tarnish products. The exposure of copper,to 75 ppb sulfur dioxide at 23 degrees C and 70% relative humidity produced a Cu2O tarnish product layer containing a small amount of sulfide. Exposure of copper to 75 ppb sulfur dioxide and 120 ppb nitrogen dioxide at 23 and 70% relative humidity produced cupric species, CuO and Cu(OH)(2), in addition to Cu2O. Trace amounts of sulfide and oxy-sulfur species were also detected.