In situ x-ray photoelectron spectroscopy was used to study the oxidation of niobium films covered by some monolayers of cerium. Significant amounts of Nb2O5 are formed at pressures as low as 6.6 × 10-6 Pa, promoted by the cerium overlayer. This catalytic activity is related to the trivalent-to- tetravalent valence change of the cerium during oxidation. The kinetics of Nb2O5 formation beneath the oxidized cerium shows two stages; the first is fast growth limited by ion diffusion, the second is slow growth by electron tunneling. © 1984 The American Physical Society.