The liquid-phase addition of methanol (MeOH) to isobutene (IB) to give methyl tert-butyl ether (MTBE) on the ion-exchange resin Bayer K2631 has been studied. Rate data were obtained free of mass-transfer influence in a continuous differential reactor operated at 1.6 MPa and 45-90 degrees C. As IB-MeOH mixture behaves non-ideally, kinetic analysis has been performed by using the UNIFAC activities. Factors, other than the non-ideality of the liquid phase, have been found to play a decisive role in the catalytic activity of the ion-exchange resin catalyst. Thus, a reformulation of the kinetic equation has been proposed in which the solubility parameter of the reaction medium is included. This equation provides a good agreement of the predicted rates with the experimental ones and of the associated apparent activation energy with the literature values.