Variations of the thermal expansion coefficient of thin polyimide films as a function of film thickness (40-260 nm) have been measured by monitoring film thickness as a function of temperature using X-ray reflectivity from samples of poly(pyromellitic dianhydride oxydianiline) (PMDA-ODA) films spin-cast onto silicon wafers. The effective expansion coefficient of the film decreases with decreasing thickness and is consistent with a bilayer model in which an interfacial region of constant thickness is formed for each film. The expansion coefficient of this surface or interfacial region is less than one-third the coefficient of the interior of the films. A simple method is described whereby the thermal expansion coefficient may be determined by reflectivity.