The electrical conductivity of Gd2O3, Dy2O3 and Er2O3 nominally doped with 2 mol% CaO, has been measured as a function of P-H2O (3 X 10(-5)-2 X 10(-2) atm) and P-O2 (1-10(-20) atm) in the temperature range 500-1200 degrees C. A defect structure with protons and oxygen vacancies compensating the acceptor dopants has been used to model the total conductivity in terms of partial conductivities. This allows determination of pre-exponential terms and enthalpies of mobilities and equilibrium constants for the formation of defects. The CaO-doped rare earth oxides are mainly protonic conductors at high P-H2O and low P-O2 and temperature. The reaction H2O(g)+V-o(")=2H(i)(’)+O-o(x) has a negative enthalpy change, and thus oxygen vacancies are the dominant ionic charge carriers at high temperatures and protons at lower temperatures. The enthalpy for the reaction is increasingly negative with increasing molar density, which, in turn, is a measure of the enthalpy of formation of the Ln(2)O(3) oxides and of the enthalpy of formation of oxygen vacancies.