The effect of relative humidity and temperature on the refractive index and thickness of thin glassy polymer films (l ∼ 500 nm) supported on silicon substrates was measured using ellipsometry. The polymers considered, polysulfone and a polyimide, exhibited increasing refractive index and film thickness with increasing relative humidity. The effect of exposure to high water activity on dry glassy polymer film properties was studied. The specific refraction, as used in the Lorentz-Lorenz equation, was determined directly for these polymers, and its dependence on temperature and aging history was examined. Water vapor sorption in thick polymer films (l ∼ 100μm) was measured using gravimetric techniques and shown to be consistent with the water vapor sorption measured in thin films using ellipsometry. The thin film water vapor sorption data are compared to the extreme possibilities of volume additivity and constant volume as water is sorbed; the results fall in between these limits but closer to the constant volume case.