Although the catalytic oxidation of inorganic molecules, as well as the oxidation and oxidative dehydrogenation of hydrocarbons, has been the subject of extensive research, the role of oxygen in surface reactions remains uncertain. Various forms of adsorbed oxygen have been proposed on the basis of kinetic and adsorption data or electrical conductivity measurements; yet, very little direct spectroscopic evidence is available apart from that provided by electron spin resonance (ESR) spectroscopy.