Electrical conductivities of Gd-doped (0.15 mol %) and nominally pure nanocrystalline CeO2-x ceramics (similar to30 nm grain size) were measured by impedance spectroscopy in the temperature range of 673-773 K under various oxygen partial pressures (1-10(5) Pa). The ionic and electronic contributions were separated using electrochemical polarization with an electronically blocking electrode, yttria-stabilized zirconia. The results allow for a clear distinction between potential explanations. It is shown that the space charge model (space charge zones with potential of similar to0.3 V resulting in depletion of oxygen vacancies and accumulation of conduction electrons) explains all the experimental features.