The formation of interfacial silicon oxide accompanying the deposition and annealing of zirconium oxide films on Si(100) has been examined. Plasma sputtering of either ZrO2 or Zr was used and, prior to analysis by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), the resulting films were exposed to ambient air and/or were annealed in O-2 or N-2. Oxygen-containing species produced by sputtering the ZrO2 target caused oxidation of bare Si(100) during ZrO2 deposition. Silicon oxidation was reduced but not eliminated, and the ZrO2 deposition rate decreased as the plasma pressure of Ar was increased from 25 to 750 mTorr. Sputtering the Zr target led to some zirconium silicide at the interface between Si and Zr. Exposure to air at 300 K oxidized the silicide and the Zr leaving a thin interfacial silicon oxide layer (< 0.3 nm). Deposition of zirconium on Si(100) covered by similar to 1.3 nn of native oxide was accompanied by an oxidation-reduction process, Zr was oxidized and Si was reduced, resulting in less than 1.3 nm of silicon oxide. For all the films, the thickness of the interfacial silicon oxide layer increased during thermal annealing in either O-2 or N-2. All the films were amorphous.