The diffusion of methanol, ethanol and toluene in a Nylon 12 and a poly(vinylidene) fluoride (PVDF) was measured using Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy, a technique which allows the quantification of the penetrant concentration diffusing in thin polymer films in situ. Furthermore, it is also possible to study multi-component diffusion using FTIR-ATR provided each component displays unique infrared absorbance peaks.Thin films of a Nylon 12 and PVDF were made by both solvent casting from either 1,1,1,3,3,3, hexafluoro-2-propanol or dimethylacetamide and by cast film extrusion. The films made by the cast film extrusion process had a more uniform thickness than those solvent cast. The diffusion coefficients of methanol, ethanol and toluene in Nylon 12 and PVDF were determined gravimetrically and by the FTIR-ATR technique. Both methods afforded comparable diffusion coefficients for similar solvent-polymer systems. The role of hydrogen bonding in these systems is also discussed. The percentage mass uptake at equilibrium of methanol, ethanol and toluene was greater for Nylon 12 than for PVDF.