The crystal structure, hydration and ionic conductivity of the inherently oxygen deficient La2Ce2O7 system have been investigated. On the basis of Rietveld analysis of neutron diffraction data, the material is found to adopt a cation disordered oxygen-deficient fluorite structure. Impedance spectroscopy, performed in the temperature range 1000-200 degrees C and as a function of water vapor and oxygen partial pressure, suggests that oxide ion conductivity dominates at high temperatures, while protons are the main charge carrier at temperatures below approximately 450 degrees C. Proton conductivity was confirmed by isotope shifts under H2O and D2O. The dissolution of water was measured by means of thermogravimetry (TG). A defect chemical model is developed to derive hydration thermodynamic parameters based on TG and conductivity data. The hydration enthalpy was, moreover, determined directly by simultaneous TG and differential scanning calorimetry (TG-DSC). The TG-DSC values were in good agreement with those modeled from conductivity and TG data. (C) 2012 Elsevier B.V. All rights reserved.