Ultrahigh-vacuum (UHV) studies show that UV irradiation (193 nn) of carbon tetrachloride adsorbed on a MgO surface produces phosgene both with/without coadsorbed water. Isotope studies show the oxygen required for the phosgene formation comes from the MgO lattice in the absence of water and from the water when coadsorbed. The reaction kinetics differ strongly between the wet and dry conditions. The observations suggest that UHV studies can reproduce important features seen in bulk studies of oxides for destruction of environmental hydrocarbons and that surface reactions may play a more important role in air or aqueous photocatalytic destruction of hydrocarbons than typically assumed.