Although Au (100) does not adsorb oxygen at either 295 K or 80 K, a barium modified Au(100) surface is active in oxygen dissociation resulting, through surface diffusion of oxygen adatoms, in the formation of a chemisorbed oxygen adlayer. This oxygen species is inactive for ethene oxidation, as is the oxygen species pre-adsorbed at an Au(100)-Ba surface at 80 K, and the clean Au(100)-Ba surface. However, when molecularly adsorbed ethene present at a Au(100)-Ba surface at 80 K is exposed to dioxygen and warmed to 140 K, surface carbonate is observed. We conclude that a transient oxygen species is the oxidant.