The authors report ellipsometrically determined optical properties of epitaxial cadmium oxide thin film grown by metal-organic vapor phase epitaxy on r-plane sapphire substrate. The ellipsometric data were collected from 0.74 to 6.45 eV with the sample at room temperature. Artifacts from the surface overlayers were reduced as far as possible by the premeasurement surface treatment procedures. Complex dielectric function epsilon = epsilon(1)+i epsilon(2) and refractive index N=n+ik spectra were extracted from multilayer modeling of the data with the B-spline functions. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3498755]