The isothermal structural relaxation of glassy, spin-cast polymer thin films has been investigated. Specifically, the thickness h of freshly cast poly(methyl methacrylate) thin films was measured over time using spectroscopic ellipsometry. The spin-cast films exhibit a gradual decrease in thickness, which is attributed to structural relaxation of the glass combined with simultaneous solvent loss. In all cases, h was found to be greater than the equilibrium thickness h ϱ , which is obtained by cooling slowly from the melt. It is observed that both the rate of the volume relaxation and the fractional departure from h ϱ (referred to as ␦ 0 ) increase with increasing film thickness. In the limit of very thin films, the initial h is close to h ϱ , and ␦ 0 is small, whereas in thick films ͑Ͼ500 nm͒, a plateau value of ␦ 0 of 0.16 is observed, which is close to the volume fraction of the solvent at the vitrification point. This dependence of ␦ 0 on thickness is observed regardless of the substrate, polymer molecular weight, or angular velocity during spin casting. Enhanced mobility near film surfaces could be leading to greater relaxation in thinner films prior to, and immediately after, the vitrification of the polymer during the deposition process.