Cross-linked epoxy network films were cast onto silicon wafers with a variety of surface treatments. X-ray reflectivity was used to characterize their electron density and thermal expansion in the rubbery state. A transition from "bulk" to "confined" expansion occurs in the range of 200-400 Angstrom, where thinner films exhibit smaller rubbery expansion coefficients. The thermal expansion behavior of the epoxy films was independent of the substrate surface treatment, which varied in both surface energy and the strength of bonding interactions with the polymer. The thickest epoxy films displayed typical rubbery thermal expansion values for temperatures above the bulk polymer glass transition temperature. The thinnest epoxy films (<120 Angstrom) exhibited typical glassy expansion values even at temperatures 20-40 degreesC above the bulk polymer glass transition temperature, independent of the surface treatment.