Equilibrium constants for the liquid-phase addition of 2-propanol to isobutene to give isopropyl tert-butyl ether (IPTBE) were determined experimentally in the temperature range 303-353 K and at 1.6 MPa. To reach etherification equilibrium, the macroporous sulfonic resin Bayer K-2631 was used as the catalyst. The UNIFAC estimates of activity coefficients were used to describe the liquid-phase nonideality. The thermodynamic equilibrium constants and the enthalpy, free energy, and entropy changes of the reaction were given as a function of temperature. At 298.15 K, the standard molar reaction enthalpy Delta(r)H(m) degrees(298.15 K) is -(22.9 +/- 1.3) kJ . mol(-1), in agreement with literature data, and the standard molar reaction entropy Delta(r)S(m) degrees-(298.15 K) and free energy Delta(r)G(m) degrees(298.15 K) are -(60.3 +/- 0.5) J .(mol . K)(-1) and -(4.9 +/- 1.4) kJ . mol(-1), respectively. Finally, at 298.15 K the standard molar enthalpy of formation Delta(f)H(m) degrees(1, 298.15 K) and the standard molar entropy of IPTBE S-m degrees(1, 298.15 K) were estimated to be -(378.5 +/- 2.7) kJ . mol(-1) and 337.6 +/- 1.5 J .(mol . K)(-1), respectively.