The liquid-phase addition of ethanol to isobutene to give tert-butyl ethyl ether (ETBE) on the ion-exchange resin Lewatit K2631 has been studied in a calorimetric reactor. The heat capacity of ETBE and the enthalpy change of the ETBE synthesis reaction in the temperature range 312-333 K have been determined. ETBE heat capacity in the liquid phase has been found to follow the equation C-p (J . mol(-1). K-1) = 486.73 - 2.253 (T/K) + 0.00479 (T/K)(2). At 298 K the standard molar reaction enthalpy is Delta H degrees = -32.0 kJ . mol(-1). A determination of the apparent activation energy of 86.5-89.2 kJ . mol(-1) has been performed graphically from the plots of heat now rate versus time. An Eley-Rideal mechanism, with two active sites involved in the rate determining step, has been proved to be correct. From this model an apparent activation energy of 80.6 kJ . mol(-1) is deduced. A -3.0 kJ . mol(-1) value has been found for the adsorption enthalpy of ethanol. This allows the estimation of the actual gel-phase activation energy of 77.6 kJ . mol(-1).