In this study, time-resolved Fourier transform infrared, attenuated total reflectance (FTIR-ATR) spectroscopy was used to characterize the diffusion of acetic acid in polyisobutylene (PIB) at different vapor activities in order to understand complex diffusion mechanisms and probe molecular structures. The relationship between the carbonyl (C=O) stretch at 1715 cm(-1), which represents a cyclic dimer, and an unknown structure at 1726 cm(-1) was investigated during the diffusion process to determine the structure of the unknown species. A local equilibrium association model suggested that the unknown species was a linear dimer in equilibrium with the cyclic dimer, A mathematical model for diffusion with isomerization, assuming local equilibrium, predicted that each species would have the same effective diffusion coefficient, but the individual diffusion coefficients could not be separated from the expression for the effective diffusion coefficient. Numerical simulations of the two coupled continuity equations were performed to show that the individual diffusion coefficients can be separated and that they are equal to each other for the acetic acid/PIB system. The values of the individual diffusion coefficients are consistent with the structural. hypothesis from the local equilibrium assumption.