From sorption experiments in literature it is known, that the solvent diffusion coefficient in nanoscale polymer layers decreases compared to its value in thicker films due to an increasing influence of the substrate. However, it is unclear whether this behavior is only related to thickness variation or also to concentration dependency, inadequate measurement routines, nonconsidered influence of phase-equilibrium or unsuitable data analysis. Here, we describe a measurement setup on the basis of a quartz crystal microbalance, that allows for a clear differentiation between the parameters thickness and concentration. For the material system poly(vinyl acetate)-methanol, sorption experiments with dry film thicknesses ranging from 50 to 685 nm on SiO 2 and gold surfaces were conducted and analyzed using different evaluation methods. A comparison to micrometerscale data (30 μm) reveals that the phase equilibrium does not vary with film thickness, but the diffusion coefficient decreases by orders of magnitude due to both thickness (10 −13 to 10 −16 m 2 /s) and concentration (10 −11 to 10 −13 m 2 /s). On the basis of the different evaluation methods we propose a model that divides the diffusional characteristics in two zones: a top layer with bulk-like behavior and a substrate-near region where solvent diffusion is significantly slower.