The chemical equilibrium of the reactive system methanol + alpha -methyl-styrene double left right arrow methyl cumyl ether was studied in the liquid and gaseous phase in the temperature range 313-328 K using a static vapour-liquid equilibrium (static VLE) method. A cation exchanger resin immersed in the liquid phase was used as heterogeneous catalyst. Equilibrium concentrations were measured chromatographically in the liquid and the vapour phase. From these data activity coefficients and thermodynamic equilibrium constants in the liquid K-a and vapour phase K-p were determined. Plots of In K-a and In K-p versus 1/T result in straight lines, from which the enthalpy of reaction Delta H-r(m)0 of methyl cumyl ether (MCE) synthesis in the liquid and gaseous phase was obtained. The consistency of these results was verified by comparison with the data of the reaction enthalpy calculated from the formation enthalpies of the reaction participants obtained by combustion calorimetry. Since the enthalpies of formation and vapourisation of methanol and alpha -methyl-styrene (alpha -MS) are known from the literature, the investigation was focused on the MCE. The standard molar enthalpy of formation Delta H-f(m)0(I) for MCE at 298.15 K was measured by means of a precise combustion calorimeter. The standard molar enthalpy of vapourisation of MCE was obtained from the temperature dependence of the vapour pressure measured by using the transpiration method. Values obtained for the enthalpy of reaction Delta H-r(m)0 of the MCE synthesis in the liquid and gaseous phase were found to be in excellent agreement with the values derived from the chemical and phase equilibria study, indicating internal thermodynamic consistency of the whole procedure.