In this study, the electrical conductivity of undoped nanocrystalline thin films of cerium dioxide was studied by impedance spectroscopy in the temperature range from room temperature to 350 S C in different atmospheres. Two types of films were studied: sample 1, a strongly oriented columnar film (thickness 75 nm); and sample 4, a less oriented 125 nm thick film. Except in one case, the impedance response of thin films, incorporated in the Pt/(n-100) Si/CeO2- x/Pt multilayer structure, consisted of a single relaxation time. Conductivity and dielectric properties of both types of films were derived from an equivalent circuit consisting of a resistance in parallel with a constant phase element. The conductivity of CeO2-x thin films was thermally activated with activation energies from similar to 0.3 to similar to 0.5 eV, and did not depend on the type of film. The absolute value of conductivity of the less oriented film (sample 4) was found to be two to three orders of magnitude higher than that of the highly oriented film (sample 1). (c) 2006 The Electrochemical Society.