This article is a reply to the opinion that the decay of stone on the Cathedral of Cologne is due to air pollution. From the history of conservation of this building it is obvious that natural weathering, primarily frost action, is responsible for the decay. There are numerous documents from the Middle Ages to the 18th century about extensive repairs on the facade. The cathedral was in such bad condition in 1800, that it was proposed it be pulled down. (Only the argument that the Cathedral of Cologne is a national monument prevented these plans.) Schinkel, a famous German architect, had to investigate the condition of the stonework, and in a detailed report in 1816, he describes all phenomena of natural weathering. In the middle of the 19th century, the work on the cathedral was continued; large parts of the completely weathered facades had to be broken down. If there is rapid decay now, this is due to the inferior stones used in the 19th and 20th century, to the high amount of sulfates (now attributed to air pollution) which was actually caused by the extensive use of concrete in the walls, and finally by the questionable restoration techniques of the architects responsible, who used stones which are resistant to air pollution but not to natural weathering. J.R. -- AATA