Electrical characterization of nominally undoped LaVO4, La0.99Ca0.01VO4-delta and La0.95Ca0.05VO4-delta was performed in various partial pressures of oxygen, water vapor and hydrogen isotopes, from 300 to 1100 degrees C by impedance spectroscopy, AC conductivity measurements (10 kHz) and EMF-measurements. XRD, SEM and EDS were used for structural, micro structural and compositional analysis. Acceptor doped LaVO4 is a pure ionic conductor in oxidizing atmospheres in the entire measured temperature range; dominated by proton conductivity at low temperatures (T < 450 degrees C) under wet conditions and oxide ion conductivity at high temperatures. A maximum in the partial proton conductivity of similar to 6 x 10(-5) S/cm was reached at 900 degrees C (p(H2O) approximate to 2.5 . 10(-2) atm). Thermodynamics of hydration and transport parameters for charge carriers in La0.99Ca0.01VO4 were derived from relations between defect chemistry, transport properties and the measured conductivity data and revealed: Delta S-hydr(0) = -130 +/- 10 J/mol K, Delta S-hydr,corr(0) = -142 +/- 10 J/mol K, Delta H-hydr(0) = -110 +/- 10 kJ/mol, mu(0,H+) = 50 +/- 6 cm(2) K/Vs, Delta H-mob,H-H+ = 75 +/- 10 kJ/mol, mu(0,vo center dot center dot) = 120 +/- 20 cm(2) K/Vs and Delta H-mob,H-vo center dot center dot = 85 +/- 10 kJ/mol. The tetrahedron (XO4) volume and migration enthalpy were found to be correlated for the series of monoclinic LaXO4. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3594145] All rights reserved.