Previous studies have documented the dramatic effects of film thickness, at least compared to the bulk state, and the more modest effects of aging temperature and polymer structure on physical aging of thin glassy polymer films. In this paper, these results are interpreted in terms of the free volume calculated from the refractive index, determined by ellipsometry, using the Lorentz-Lorenz equation. The relative progress of aging towards the final equilibrium state was determined using this approach. The free volume data were fitted to the Struik model for the self-retarding volume contraction on aging to obtain the thickness and temperature dependent parameters. The gas permeability versus aging time is related to the free volume computed from the refractive index. The effect of prior history on the aging of thin glassy polymer films was briefly explored. (c) 2005 Elsevier B.V. All rights reserved.