The influence of film thickness and molecular weight on the diffusion coefficients of water, benzene, and trifluoroacetic acid in two photoresist polymers, poly(p-hydroxystyrene) and bis-trifluoromethyl carbinol substituted poly(norbornene), has been studied using quartz crystal microbalance (QCM) methods. Diffusion coefficients for films as thin as approximately 50 nm were determined. It was observed that the diffusion coefficient was a strong function of film thickness, and that the diffusion coefficient decreases drastically as film thickness is reduced below a critical value. This critical thickness value is found to be a function of both polymer structure and molecular weight. In addition, the effect of film thickness on the equilibrium uptake of the various penetrants was also determined. In particular, the equilibrium water uptake was shown to depend strongly on film thickness, polymer structure, and polymer molecular weight.