The thicknesses of thin oxide films grown on silicon in oxygen were measured by transmission electron microscopy (TEM), ellipsometry, and profilometry. For films thicker than about 30 nm the thicknesses measured with these three techniques were the same within experimental error; the ellipsometric thicknesses were calculated from a model of a single oxide film with a refractive index that did not vary with thickness. For films thinner than 30 nm, the ellipsometric thicknesses calculated from this model were greater than those measured by TEM, as found by others. Ellipsometric DELTA and psi angles were measured for a number of films thinner than 30 nm in four liquids of different refractive index, from 1.33 to 1.495. These results are not consistent with a model of a single film of refractive index increasing with decreasing thickness, as deduced by several other workers from statistical fits to ellipsometer data. Good agreement of measured DELTA and psi values for films whose thicknesses were determined by TEM was found with calculated DELTA and psi values for a model of two films, including the main oxide film of constant refractive index, and an interfacial film of constant refractive index and thickness of 1.2 nm. These results suggest that reliable thicknesses of oxide films of silicon below 30 nm thick can be calculated from such a model.