The hydration reaction of Li β-alumina has been studied using infrared measurements of large single crystals equilibrated at controlled water vapor pressure (2–80 kPa) and temperature (250–450 °C) conditions. The equilibrium H2O concentrations in the conduction plane can be carried continuously and reversibly up to a saturation limit of about 1.8 H2O per unit cell. Values of the enthalpy and entropy for the hydration reaction at 300–450 °C were determined to be ΔH=−102 kJ/mol and ΔS= −167 J/mol K. In Li β-alumina, some of the H2O dissociates to form OH− ions and H+ ions which are bound to lattice oxygen ions; the enthalpy for the dissociation reaction is ∼10 kJ/mol. In addition to OH−, H3O+ and H5O+2 species have been identified from the observed change in the equilibrium concentration with the water vapor pressure. The relative concentrations of the different types of OH− species depend on the sample age. The hydration enthalpy for Na β-alumina of ∼−84 kJ/mol was determined from a similar study.