The ac conductivities of Er6WO12 and 1% Ca-doped Er6WO12 have been characterized as a function of oxygen and water vapor partial pressures in the temperature range 300 - 1200 degrees C. Partial conductivities have been determined from the open-circuit voltage of p(O-2), p(H-2), and p (H2O) concentration cells. The materials exhibit n- and p-type electronic conductivity and oxygen ion and proton conductivity, depending on the conditions. Protons are the major ionic charge carriers in wet atmospheres below 1150 C. Hydration thermodynamics and transport parameters have been determined under conditions where the acceptor doping is predominantly compensated by oxygen vacancies and protons. (c) 2006 The Electrochemical Society.