We evaluate in this work the effect of Gd substitution on the dc activation energy, E-dc, and ionic conductivity of the pyrochlore-type gadolinium zirconate, P-Gd2Zr2O7, Several compositions with the general formulae Gd-2 (y)Ln(y)Zr(2)O(7) (Ln = Er3+, Y3+, Dy3+. Sn3+, Nd3+ and La3+) were prepared by mechanically milling and firing stoichiometric mixtures of the corresponding elemental oxides and their electrical properties analyzed as a function of frequency and temperature by using impedance spectroscopy. Whereas Cd substitution in P-Gd2Zr2O7 by smaller lanthanides induces a pyrochlore to fluorite phase transition, using larger dopant cations yields partially disordered pyrochlores. Despite of higher structural disorder, ionic conductivity values measured for the fluorite-type materials are lower than those observed for pyrochlores whereas activation energies for oxygen migration in the series decrease monotonically as the average size of the A cation increases. (c) 2008 Elsevier B.V. All rights reserved.