The glass transition temperatures (Tg's) of ultrathin films (thickness 80-18 nm) of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were measured on surfaces with interfacial energies (gamma (SL)) ranging from 0.50 to 6.48 mJ/m(2). The surfaces consisted of self-assembled films of octadecyltrichlorosilane (OTS) that were exposed to X-rays in the presence of air. Exposure to X-ray radiation systematically modified the OTS by incorporating oxygen-containing groups on the surface. The interfacial energy for PS and PMMA on the OTS surface was quantified as a function of X-ray dose using the Fowkes-van Oss-Chaudhury-Good model of surface tension. The T-g values of the films were characterized by three complementary techniques: local thermal analysis, ellipsometry, and X-ray reflectivity. Within the resolution of the techniques, the results were in agreement. At low values of gamma (SL), the T-g values of PS and PMMA films were below the respective bulk values of the polymers. At high values Of gamma (SL), the T-g values of PS and PMMA films were higher than the bulk values and increased monotonically with increasing gamma (SL). The deviation of the Tg values of the films compared to the bulk values increased with decreasing film thickness. For a specific film thickness of PS and PMMA, the difference between the Tg of the film and T-g of the bulk polymer (DeltaT(g) = T-g(film) -T-g(bulk)) scaled linearly with gamma (SL) irrespective of the chemistry of the polymer.