The electrical and defect chemical properties of acceptor doped langasite have been investigated over wide ranges of PH2O, Po-2, and temperature. All compositions are pure proton conductors up to 800 degrees C in wet atmospheres and mixed oxide ion-p-type conductors at higher temperatures. The enthalpy of mobility of protons is 75 +/- 3 kJ/mol, while that of oxygen vacancies is 125 +/- 7 kJ/mol.The standard enthalpy and entropy of hydration are -100 +/- 3 kJ/mol and -157 +/- 5 J/mol K, respectively. Langasite based sensors may therefore be affected by dissolution of protons from H2O in the bulk crystal lattice up to temperatures as high as 1000 T. (C) 2014 Elsevier B.V. All rights reserved.