Abstract The mobility of ethyl acetate (EAc) as a function of crystallinity in poly(vinylidene fluoride) (PVF 2) has been quantified utilizing methodology based on the recently developed rheo-photoacoustic Fourier transform infrared spectroscopy (RPA FTi.r.). The diffusion measurements obtained with the photoacoustic cell, combined with the independent sorption and X-ray measurements, indicate that the crystalline phase of the polymer is impermeable to EAc. Spectroscopic RPA FTi.r. measurements agree with the independently determined diffusion coefficients and the theoretical predictions relating diffusion to the per cent crystalline phase in PVF 2. This study indicates that RPA FTi.r. methodology is highly sensitive to the gas phase analysis and, with the use of proper calibration procedures, can be applied to a quantitative analysis of diffusion in polymers.