We report investigations of the glass transition temperature and the thermal expansion behaviour of 25-10 mu m thick PMMA films coated on Si wafers. A spectroscopic ellipsometer equipped with an oven, temperature control and data analysis fully integrated into the ellipsometer software was used. For films above 50 nm we found within experimental error the same T-g and thermal expansion behaviour of the PMMA films. Deviations found for thinner PMR IA films on Si are attributed to the temperature dependence of the optical properties of the Si-substrate.