The ultimate aim of the polarity driven kinetics project is to develop a comprehensive model that is able to predict heterogeneously catalysed reaction kinetics in the liquid phase over a wide range of conditions. Phase 1 concerns the esterification of acetic acid with methanol using benzenesulfonic acid as homogeneous and the ion exchange resin Lewatit K1221 (containing the same acid sites) as heterogeneous catalyst, in the presence and absence of THF solvent. The kinetics follow an Eley-Rideal mechanism with additional competitive interactions with the acid site. The activity of the liquid phase components was evaluated using the NRTL model and by the Flory-Huggins model in the polymer phase. Methanol and water are absorbed most strongly, followed by THF, methyl acetate, and acetic acid. The swelling of the resin up to 3.5 and the differences between ions also influence the kinetics, which is not yet accounted for. This follows in Phase II.