Experiments on the oxidation, at room temperature, in 5×10-8 Torr oxygen, of vapor deposited films of cerium, niobium, and niobium covered by some monolayers of cerium are reported. The valence-band structure was studied by ultraviolet photoemission spectroscopy (UPS), the surface potential due to chemisorbed oxygen was measured by measuring work-function change by UPS, and oxide growth was followed by x-ray photoelectron spectroscopy. The energy level a of oxygen chemisorbed on CeO2 was determined from UPS difference spectra and found to be at 4 eV below the Fermi level F. Comparing this with measured values of the surface potential (VS=-0.6 V) we conclude that contrary to earlier predictions, the entire value of the Mott potential VMe-1(F-a) does not materialize as a surface potential in the course of oxidation. We found, that for the oxidation of Nb the incorporation of oxygen ions at the surface is the rate-limiting step, while in case of cerium, and niobium covered by some monolayers of cerium, the first stage of oxidation is limited by oxygen-ion diffusion, and the second stage of oxidation is limited by electron tunneling. © 1985 The American Physical Society.