The electrical conductivity of 2 mol% CaO-doped Tb2O3 has been characterized as a function of the oxygen pressure and water vapor pressure from 300 to 1200 degrees C with two- and four-point electrode setups. EMF-rneasurements under reducing conditions have, furthermore, been applied to establish the contribution from the different charge carriers. Tb2O3 is a p-type semiconductor. Protons are the major ionic charge carrier under reducing conditions constituting 2-35% of the total conductivity, increasing with decreasing temperature. The conductivity under wet hydrogen has been modeled assuming that protons and oxygen vacancies compensate the acceptor (Ca). On this basis, the enthalpy and entropy of hydration, and the enthalpy and pre-exponential of proton mobility have been determined. These parameters are compared to corresponding data for the other rare-earth sesquioxides. (c) 2005 Elsevier B.V. All rights reserved.