Thermodynamic equilibrium constants for the liquid-phase synthesis of ethyl terl-butyl ether (ETBE) were determined experimentally in the temperature range 40-80°C (313-353 K) at 1.6 MPa for an initial molar ratio of ethanol to isobutene ranging from 1 to 1.5. To reach etherification equilibrium a macroporous sulphonic acid resin (K-2631, Bayer) was used as a catalyst. The thermodynamic equilibrium constant and the enthalpy, free energy, and entropy changes are given as a temperature function. At 298 K, ΔH° = −34.8 kj mol−1, ΔG° = −11.7kJmol−1, and ΔS° = −77.3J mol−1K−1. A comparison with the values obtained for MTBE is also included.