The optical constants of Sb and SbOx films grown by d.c. sputtering are determined over a wide spectral (270-870 nm) and composition (x =0.12-2.0) range. Spectroscopic ellipsometry is used to determine the film optical constants. The composition of the films is obtained by combining nuclear reaction analysis and Rutherford backscattering spectrometry. Optical properties of the films change drastically at a critical oxygen content (x = 0.19) which is related to the transition from a crystalline Sb-like film with optical properties similar to crystalline Sb to an amorphous Sb-like film. It will also be shown that amorphous SbOx films behave optically as a mixture of amorphous Sb-like material and a stoichiometric Sb oxide (Sb2O4, x = 2.0).