The ionic and electronic conduction of Gd0.95Ca0.05NbO4 has been determined with ac conductivity measurements and impedance spectroscopy in combination with transport number measurements using the open circuit voltage (electromotive force) of concentration cells. Depending on the reaction conditions, protons, oxygen ions, and electron holes are the main contributions to the total conductivity. The conductivity data have been modeled according to a point-defect structure where the acceptor, Ca, is predominantly compensated by oxygen vacancies and protons. Thermodynamic constants have been derived for the hydration of the oxygen vacancies and the transport properties of oxygen ions, protons, and electron holes. (c) 2006 The Electrochemical Society.