The electrical properties of bulk and grain boundaries of scandia-stabilized zirconia co-doped with yttria and ceria have been determined as a function of temperature (300 < T/degrees C <700) and oxygen partial pressure [10(-24) <= p(O(2))/bar <= 1. T=700 degrees C] by application of impedance spectroscopy. The yttria and ceria contents of Ce(x)Y(0.2) (-) (x)Sc(0.6)Zr(3.2)O(8) (-) (delta) (0 <= x <= 0.2) have been varied systematically. Homogeneous samples have been prepared by means of a sol-gel (glycine-nitrate) combustion process. The ionic conductivity in air is almost independent of composition with typical values around 0.03-0.04 S cm(-1) for the bulk at 700 degrees C. A significant decrease of the ionic conductivities of bulk and grain boundaries is found for samples co-doped with ceria at low oxygen partial pressures [p(O(2)) <10(-15) bar, T=700 degrees C]. Activation energies for the ionic transport in oxidizing (air) and reducing (1%-H(2)/Ar) atmospheres have been extracted from Arrhenius-plots. The oxygen nonstoichiometry in 1%-H(2)/Ar has been investigated by employing thermogravimetry. The decrease of the ionic conductivity under reducing conditions is accompanied by an increase of the corresponding high temperature activation energy of the bulk, which is interpreted in terms of defect association or clustering. (C) 2010 Elsevier B.V. All rights reserved.