The optical functions of titanium dioxide (TiO2). tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2) have been determined in the spectral range from 1.5 to 5.4 eV (wavelength range from 230 to 840 nm). The ellipsometric spectra of 200 nm thick layers sputtered on a glass substrate were measured by a four-zone null spectroscopic ellipsometer. The data have been fitted by a Tauc-Lorentz model recently derived by Jellison and Modine for the optical functions of amorphous materials. The model dielectric function is based on a combination of the Tauc band edge and the Lorentz oscillator. The effects of the surface and interface layers and layer inhomogeneity on the measured data are discussed.