Bulk conduction and relaxation of the [(ZrO2)(1-x)(CeO2)(x)](0.92)(Y2O3)(0.08) (0 <= x <= 1) solid solutions were studied using impedance spectroscopy at intermediate temperatures (200-500 degrees C). The bulk conductivity as a function of x shows a "V-shape" variation which is a competitive effect of the defect associates and the lattice parameter. In the ZrO2-rich region (x<0.5) CeO2 doping increases the concentration of defect associates which limits the mobility of the oxygen vacancies: in the CeO2-rich region (x > 0.5) the increase of x increases the lattice parameter which enlarges the free channel for oxygen vacancy migration. Further analysis indicates the ionic radius of the tetravalent dopant determines the composition dependence of the ionic conductivity of the solid solutions. When doping YSZ with other tetravalent dopant with similar ionic radius with Zr4+, e.g., Hf4+, such "V-shape" composition dependence of the bulk conductivity cannot be observed. (C) 2011 Elsevier B.V. All rights reserved.