Ta2O5 filMS, formed on Si(100) by metallorganic chemical vapor deposition using pentamethoxy tantalum as a metallorganic source and O2 as reactant gas at deposition temperatures of 473-773 K, were analyzed by ellipsometry, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The ellipsometric data may be explained by a double-layer optical model which presumes a thin interface layer between the Ta2O5 film and the Si substrate. Ta2O5, SiO2, Si, and a lower oxide TaO(x) were detected at the interface region between the Ta2O5 film and the Si substrate by XPS. Cross-sectional TEM of the interface layer formed at 773 K showed that it was 6.1 nm in thickness and was amorphous. The growth rates of Ta2O5 film and interface layer increased with increasing deposition temperature.