The liquid-phase addition of methanol to isobutene to give tert-butyl methyl ether (MTBE) on the ion-exchange resin Lewatit K2631 has been studied in a calorimetric reactor. Heat capacity of MTBE and enthalpy of the MTBE synthesis reaction in the temperature range 312-333 K have been determined. MTBE heat capacity in the liquid phase has been found to obey the equation c(P) (J.mol-1.K-1) = 472.34 - 2.468(T/K) + 0.005071 (T/K)2. At 298 K the standard molar reaction enthalpy is DELTAH-degrees = -33.8 kJ.mol-1. A method to estimate apparent activation energies from heat flow rate in a given reaction has been developed and proved to be valid for the MTBE synthesis. Using this method, an apparent activation energy of 91.1-95.2 kJ.mol-1 is calculated. A -3.8 kJ mol-1 value has been found for the adsorption enthalpy of methanol on the ion-exchange resin Lewatit K2631 by a combination of reaction calorimetry and thermogravimetry. This allows the calculation of an activation energy on the gel phase of the resin of 91 kJ mol-1.