The dilation of polycarbonate (PC), poly(vinyl acetate) (PVAc), and poly(ether urethane) (PEUT), induced by acetonitrile sorption at 40 degreesC, was studied using FTIR-ATR spectroscopy. Results compared well to those obtained through direct observation of volumetric changes using a CCD camera. Polymer dilation and penetrant sorption kinetics were monitored simultaneously with the spectroscopic technique, allowing further mechanistic insight into these processes. For the rubbery polymers (PVAc and PEUT), a linear relationship exists between time-evolved dilation and sorption, indicating that chain relaxation is fast relative to the diffusion process. However, for the glassy polymer (PC), there is a lag in dilation relative to sorption, suggesting that the nonequilibrium microvoids are filled before significant dilation occurs in the polymer. Model predictions of the extent of dilation, using the lattice fluid model for PVAc and PEUT and the nonequilibrium lattice fluid model for PC, overestimate the experimental data, particularly for PC. The large deviation for PC is due to the extreme sensitivity of the NELF model to polymer density, which is used to reflect the nonequilibrium. nature of the glassy polymer and is strongly history dependent.