The electrical conductivity of polycrystalline cerium oxide was investigated in the nanometer and micrometer size range. Nanocrystalline samples of different grain size were prepared by uniaxial hot-pressing of nanocrystalline powder at various temperatures and pressures. Additional annealing at high temperatures was employed in order to obtain microcrystalline samples. An equivalent-circuit analysis of ac-impedance spectra based on the brick-layer model was performed and the apparent bulk conductivity determined. The effect of a variation in temperature or oxygen partial pressure revealed the rather different nature of the electrical transport properties in the nano- and microcrystalline materials. Nanocrystalline cerium oxide exhibited electronic conductivity under conditions at which, microcrystalline samples showed impurity-controlled ionic conductivity. The electronic conductivity of nanocrystalline samples was larger than the intrinsic electronic conductivity of pure single crystalline cerium oxide and was increasing with decreasing grain size. The experimental results: were analyzed according to the defect chemistry of cerium oxide and consequences of a space charge effect on the partial electronic and ionic conductivity in polycrystalline cerium oxide will be discussed.