We have studied wet thermal oxidation between 700 and 1100 degrees C of strained Si1-yCy (0 less than or equal to y less than or equal to 0.02) layers grown by molecular beam epitaxy on Si(001) substrates. The oxidation kinetics and the refractive indices of the oxides grown on Si1-yCy were monitored by ellipsometry. They show no significant differences in comparison with oxides grown on silicon. Secondary ion mass spectroscopy and x-ray photoelectron spectroscopy reveal the appearance of carbon in the oxide. To find an appropriate temperature window for oxidation, we investigated the underlying Si1-yCy layers after oxidation by infrared absorption measurements. We observed a decrease of the substitutional carbon concentration for oxidation temperatures higher than 800 degrees C. Thus, only a small temperature window for the growth of good thermal oxide on Si1-yCy layers is available. Infrared absorption measurements on the oxide vibrational modes reveal a small influence of the carbon concentration on the structural quality of the oxide.