Equilibrium constants for the liquid-phase syntheses of methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) were determined experimentally in the temperature range 313-353 K and at 1.6 MPa, using as source of 2-methylpropene (isobutylene) a C4 olefinic cut proceeding from a steam cracking unit. To reach etherification equilibrium, the macroporous sulfonic acid resin K-2631 (Bayer) was used as the catalyst. The thermodynamic equilibrium constants and the enthalpy, free energy, and entropy changes of reactions were given as a temperature function. At 298 K, the standard molar reaction enthalpy DELTA(r)H(m)-degree(298 K, 1.6 MPa) for MTBE and ETBE are -(37.3 +/- 2) and -(34.8 +/- 1.3) kJ.mol-1, respectively, and are compared with literature data. A comparison of the equilibrium constant values with those obtained using pure reactants with and without small initial amounts of water (< 5 wt %) is also included. The UNIFAC estimates of activity coefficients were used to describe the liquid-phase nonideality. The standard molar enthalpies of formation DELTA(f)H(m)-degree(1,298.2 K) of MTBE and ETBE are -313.5 and -349.9 kJ.mol-1, respectively.