X-ray photoelectron spectroscopy (XPS) has been used to characterize the oxidation of a clean Ni(Pt)Si surface under two distinct conditions: exposure to a mixed flux of atomic and molecular oxygen (O + O-2; PO+O2 = 5 x 10(-6) Torr) and pure molecular oxygen (O-2; PO2 = 10(-5) Torr) at ambient temperatures. Formation of the clean, stoichiometric ( nickel monosilicide) phase under vacuum conditions results in the formation of a surface layer enriched in PtSi. Oxidation of this surface in the presence of atomic oxygen initially results in formation of a silicon oxide overlayer. At higher exposures, kinetically limited oxidation of Pt results in Pt silicate formation. No passivation of oxygen uptake of the sample is observed for total O + O-2 exposure <8 x 10(4) L, at which point the average oxide/silicate overlayer thickness is 23 (3) angstrom ( uncertainty in the last digit in parentheses). In contrast, exposure of the clean Ni(Pt) Si surface to molecular oxygen only ( maximum exposure: 5 x 10(5) L) results in slow growth of a silicon oxide overlayer, without silicate formation, and eventual passivation at a total average oxide thickness of 8(1) angstrom, compared to a oxide average thickness of 17(2)angstrom ( no silicate formation) for the as-received sample (i.e., exposed to ambient.) The aggressive silicon oxidation by atomic oxygen, results in Ni-rich silicide formation in the substrate and the kinetically limited oxidation of the Pt. (C) 2008 Elsevier B. V. All rights reserved.