Industrial chlorofluorocarbons have now accumulated so much in the atmosphere that the ClOx radicals produced from their oxidation are causing substantial reductions in the ozone layer. Here we measure phosgene, which is one possible product from the oxidation of natural and industrial chlorinated hydrocarbons and which can oxidize further to form ClOx. Our measurements show a mixing ratio of 17 p.p.t.v. in the upper troposphere, and an average of 22 p.p.t.v. in the lower stratosphere. These values are substantially greater than those estimated with a model that only considers the photochemical breakdown of CCl4, indicating the possible significance of other more reactive chlorocarbon compounds, especially CHCl3, CH3CCl3, C2HCl3 and C2Cl4 and their oxidation products in supplying chlorine to the lower stratosphere. © 2002 Nature Publishing Group.