A method of determining equilibrium constants of catalytic reactions was described using gas chromatography. Both the equilibrium constants and enthalpies of methyl-tert-butyl ether from methanol and isobutene, ethyl-tert-butyl ether from ethanol and isobutene on catalyst Amberlyst 15 in the gas phase were determined. The enthalpy values of these reactions were compared with the enthalpies calculated from thermodynamic data and the experimental data given in the literature.