The esterification of acetic acid with methanol has been investigated using the acidic ion exchange resin Lewatit K1221 and benzenesulfonic acid as heterogeneous and homogeneous catalyst, respectively. A thermodynamic model able to fully describe the behaviour of the liquid and polymer phases has been developed. The model is based on the Flory-Huggins solution theory and explicitly accounts for the swelling pressure. The solvent-solvent interactions in the model have been determined a priori by regression of the FH model against the NRTL model. As a result, liquid phase activities calculated by a new thermodynamic model agree well with those from the NRTL model.