The oxidation of Si(100) surfaces covered with 3 AÌScaron; Ce was studied by high-resolution photoemission spectroscopy. The oxygen uptake was found to be enhanced by orders of magnitude by the presence of Ce. Because of chemical reaction between Ce and Si, a mixed oxide with a Ce:Si:O composition ratio of 1:1:3 is formed. The sharp photoemission spectra of this oxide indicate the formation of a well-defined compound in which both Ce and Si are in a 3+ oxidation state. © 1986 The American Physical Society.