The oxidation kinetics and the optical properties of the new SiO2/Si1-x-yGexCy system (grown epitaxially on Si) have been studied in detail by ellipsometry (EL) and infrared spectroscopy (FTIR). While EL is straightforwardly applied to silicon, the thermal oxidation process affects the structure and refractive index of the underlying non-oxidized Si1-x-yGexCy. Thus, to determine the oxidation kinetics, part of the oxide must be etched-away to monitor the variation of the refractive index of the Si1-x-yGexCy layer after oxidation. The EL data has been interpreted as having a Ge-rich superficial layer and beta-SiC precipitates embedded in a SiGeC matrix. This model have allowed us to correlate the EL results with the characterization performed by SIMS and TEM (Ge pile-up at the interface with the oxide and beta-SiC nanoprecipitates in the bulk of the semiconductor). These changes are driven by rejection of Ge from the oxidation front and carbon leaving its substitutional site (C-s). The inverse evolution of the C-s and beta-SiC content has been quantified by monitoring their IR bands at 607 and 820 cm(-1) respectively. Accurate results on dry thermal oxidation kinetics of Si1-x-yGexCy in the thin oxide regime are presented.