The Ce1-xGdxO2-delta (0.02 <= x <= 0.3) solid solutions with similar to 30 ppm SiO2 were prepared through the conventional mixed-oxide method from high-purity CeO2 and Gd2O3 powders. The ionic conductivity (especially the grain boundary (GB) conduction) in this system was studied as a function of dopant content, over the temperature range of 250-750 degrees C in air, using an impedance spectroscopy. The GB impedance played an important role in the total conduction in the samples with low Gd content (usually x <= 0.1), but decreased sharply with increasing Gd content. Both the total and GB conductivities maximized at the composition x = 0.15, and this composition also had the lowest activation energies for the total and GB conduction. The maximum total conductivity at 700 degrees C, sigma(t) = 4.07 (Omega m)(-1) with the activation energy, E-t = 0.77 eV, was found for the composition x = 0.15. (c) 2005 Elsevier Ltd. All rights reserved.