The roles of grain size and chemical composition on the electronic conductivity (σ) of CeO2 are reported. In undoped high purity (99.995%) CeO2, σ increases with decreasing grain size, indicating generation of low oxidation state Ce species, Ce3+ ions, which act as the electrons. In low purity undoped CeO2, the chemical impurities act as both the acceptor dopants, as well as the components (such as Si) to form amorphous phase in the grain boundaries. In low purity (99.5%) undoped CeO2, size effects are, therefore, influenced by the presence of acceptor dopants and/or blocking grain boundary phases. In low purity undoped CeO2, the grain boundary contribution to the overall resistance decreases with increasing grain size due to dilution effects of the amorphous grain boundary phases. The intentional addition of acceptors (10%) will pin oxygen vacancies over a wide range of oxygen activity. Size effects in acceptor doped CeO2 are mainly related to the presence of grain boundary phases. In Ce0.90Gd0.10O1.95, the grain boundary contribution to total resistance increases with decreasing size, which is due to trapping of oxygen ions.