The glass transition temperature, Tg, of polystyrene (PS) thin films on silicon wafers with different surface preparations was determined in order to explore the effect of interfacial interaction energy on local chain dynamics. To enhance interface behavior, the film thickness included in this work was as low as 50 Å. PS with monodisperse molecular weights was chosen as a model polymer. Two different silicon surfaces were prepared: sulfuric acid-clean and hydrogen passivated. Tg of thin films was determined by monitoring the film thickness as a function of temperature. Tgwas identified as the temperature where the thermal expansion coefficient underwent an abrupt change. X-ray reflectivity was used to determine film thickness with a precision of a few angstroms. Tg of PS was found to depend strongly on the initial film thickness, as well as the substrate surface.