Airborne acidifying pollutants are known to be one major cause of corrosion of different materials including the extensive damage that has been observed on historic and cultural monuments. In order to fill some important gaps of knowledge in this field the executive Body for the Convention on Long-range Transboundary Air Pollution decided to launch an international cooperative programme within the United Nations Economic Commission for Europe (UN ECE). The programme started in September 1987 and involves exposure at 39 test sites in 11 European countries and in the United States and Canada. The aim of the programme is to perform a quantitative evaluation of the effect of sulphur pollutants in combination with NO_x and other pollutants as well as climatic parameters on the atmospheric corrosion of important materials. The present report summarises results from the first two years of exposure of copper and bronze samples. The one-year samples were exposed from September 1987, when the project began, until September 1988. The two-year samples were exposed until September 1989. The next withdrawal of specimens will be after a period of four years (Sept. 1991). The exposure time for the last samples has not yet been determined. -- CAL