The diffusion of water into ultra-thin PFPE (perfluoropolyether) films supported on gold substrates as a function of relative humidity (RH) and of PFPE film thickness from 20 – 100 Å is investigated. The complete adsorptive wave is fit by a two-stage sorption model using a linear superposition of diffusion and polymer relaxation controlled mass uptake. The initial portion of the adsorptive wave, fit by Fickian diffusion, provides a diffusion constant, D, for water sorption as a function of RH and of PFPE film thickness. The D values are 10-18 to 10-16 m2/s depending upon RH and PFPE film thickness. The diffusion constant increases non-linearly when RH exceeds ~ 70 - 80%, indicative of a significant change in the PFPE film structure to accommodate the water sorption.