The optical properties of as-prepared and rapid thermal oxidized (RTO) heteroepitaxial Si1−x−yGexCy alloys grown on Si substrate have been characterized using spectroscopic ellipsometry. The critical points E1, E0′, E2 band gaps were determined by line shape fitting in the second derivative spectra of the pseudo-dielectric functions. For as-prepared films, the E1 gap increases with C concentration and a linear dependence on C content was observed. However, the E2 gap decreases as the C concentration increases. For the RTO samples, the amplitude of E2 transition reduces rapidly and the E1 transition shifts to a lower energy. The reduction in the amplitude of E2 transitions is due to the presence of oxide layer. A high Ge content layer and the low C content in the RTO films account for the E1 shift to lower energy and the increase of the refractive indices.