The molecular interdiffusion across a poly(vinyl pyrrolidone)/vinyl ester monomer (PVP/VE) interface was investigated in situ by Fourier transform infrared attenuated total reflectance (FTi.r.-ATR) spectroscopy. In order to separate the effects of the vinyl ester monomer diffusion and the VE crosslinking reaction, ATR experiments were carried out at temperatures below the normal curing temperature of the VE. The infrared bands at 1717 and 1507 cm(-1), characteristic of the vinyl ester monomer, and the bands at 1669 and 1419 cm(-1) characteristic of the PVP, were used in a quantitative analysis. Diffusion coefficients were determined by following intensity variations in these selected characteristic bands as a function of time, and fitting this data to a Fickian model. A mutual diffusion coefficient on the order of 2 X 10(-8) cm(2)/s was calculated at 100 degrees C. Although the magnitude of the diffusion coefficients obtained was consistent with values found in the literature for diffusion of small molecules in polymers, the simple one-dimensional Fickian diffusion model employed as a first approximation had some limitations for the particular (PVPNE) system. Because the glass transition temperature of PVP changed as diffusion proceeded, the mutual diffusion coefficient should not stay constant. In fact, it was shown that the T-g can drop by as much as 140 degrees C during the diffusion process.