The interface of Si grown epitaxially on the (100) plane of yttria-stabilized cubic zirconia (ZrO2,Y2O3) (CZS) has been investigated using XPS, LEED, and Auger depth profile analysis. Submicron thick Si films were deposited by vacuum sublimation onto heated CSZ substrates. Results show that the CSZ crystal was chemically reduced when heated in UHV, yellowing and then darkening the crystal presumably through loss of oxygen from the bulk. This reduction could be reversed by heating the crystal in O2. Initial Si depositions resulted in the formation of SiOx and further reduction of the ZrO2. After further depositions, when only elemental Si was detected, the Zr was completely reduced from its oxide state within the sampling depth of the XPS. Yttria remained unchanged throughout heating and Si depositions. Auger depth profiles showed that the SiOx layer remained at the interface even for film thicknesses greater than 200 nm. CSZ crystals with hydrocarbon contamination on their surfaces underwent chemical reaction upon heating in UHV to form zirconium carbides. Epitaxy on this surface is difficult or impossible. Removal of the carbide can be accomplished by heating in an oxygen ambient.