Films, formed on Si(100) by metallorganic chemical vapor deposition using pentamethoxy tantalum as a metallorganic source and 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 film and the Si substrate. ,, Si, and a lower oxide were detected at the interface region between the 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 film and interface layer increased with increasing deposition temperature.