The conductivity of acceptor-doped LaNbO4 has been investigated in the temperature range 300 to 1200 degrees C as a function of the oxygen pressure and water vapor pressure by means of impedance spectroscopy and EMF measurements. The conductivity is predominantly ionic below 800 degrees C in air and for higher temperatures under reducing conditions. Protons are the major ionic charge carrier in the presence of water vapor. A maximum in proton conductivity of similar to 0.001 Skin was obtained at 950 degrees C in atmospheres containing ca 2% H2O. At high temperatures (> 1000 degrees C) under oxidizing conditions, electron hole conduction prevails. The conductivity has been modeled assuming that oxygen vacancies and protons compensate the acceptor doping. Transport coefficients describing mobility of defects and thermodynamic constants for the incorporation of protons have been derived. (c) 2006 Elsevier B.V. All rights reserved.