For the first time, the defect structure of mayenite was developed on the basis of conventional Kroger-Vink method and successfully verified using available data on its properties such as total conductivity, hydration/dehydration and oxidation/reduction. As a consequence, equilibrium constants of the quasichemical reactions of mayenite reduction and hydration were determined as a function of temperature. This allowed estimation of Ca12Al14O33 (-) (delta) enthalpy of reduction and hydration. The former was found to be equal to 360 kJ/mol while the latter was estimated as -238 kJ/mol. The defect structure model proposed enabled estimation of oxygen nonstoichiometry in mayenite. (C) 2015 Elsevier B.V. All rights reserved.