Rate data are reanalyzed for MTBE synthesis in the liquid phase at 318 - 363 K on the sulfonic macroporous resin Bayer K-2631. The data were obtained in a continuous upflow packed-bed reactor fed with isobutene (IB)-methanol (MeOH) mixtures with an IB/MeOH molar ratio ranging from 0.5 to 2.0. Rate equations based on LHHW and ER mechanisms do not predict rate data accurately, because of the influence of the reaction medium on the catalyst activity. A factor evaluating the resin-medium affinity has been introduced in the rate equations which, as a result, Ft the data with a good agreement. Fitted parameters were the overall kinetic constant and the resin solubility parameter. The apparent activation energy agrees with the values quoted in the literature, and the resin solubility parameter and its temperature dependence are plausible.