Copper samples were exposed for 10 days in synthetic laboratory air at 75% relative humidity. To explore the possible influence bf ozone on the atmospheric corrosion rate of copper, various combinations of the gaseous pollutants sulfur dioxide, nitrogen dioxide, and ozone were added. Ozone promotes the oxidation of sulfur dioxide to sulfate more efficiently than nitrogen dioxide does. A synergism between sulfur dioxide and ozone is suggested. This synergism includes both the oxidation of sulfur dioxide by ozone and the capability of ozone to form oxides, hydroxides, or other oxygen-containing reaction products in the presence of smaller amounts of sulfur dioxide. The synergistic effect possibly can explain the unexpectedly high corrosion rates of copper found at rural sites within the UN ECE exposure program. The rural sites are characterized by low sulfur dioxide and nitrogen dioxide concentrations, and by high ozone concentrations.