The production of phosgene was observed when a pure carbon tetrachloride molecular beam was impinged on an oxygen modified Ir(111) surface. We propose a mechanism involving the partial surface decomposition of the CCl4 molecule to CCl2 which subsequently reacts with adsorbed oxygen to form phosgene. Phosgene formation was maximized at an oxygen surface coverage of approximately 0.3 ML. increasing the surface temperature resulted in a decrease in formation of COCl2, as competing reactions became important, especially the rapid and more complete decomposition of CCl4 on the surface prior to reaction with oxygen.