The electrical properties of CeNbO4+delta have been investigated at 1073-1223 K in the oxygen partial pressure range 10(-17) to 0.36 atm. The conductivity and Seebeck coefficient behaviour indicates that, at oxygen chemical potentials close to atmospheric, tetragonal CeNbO4+delta possesses a mixed ionic and p-type electronic conductivity. The ion transference numbers under the p(O-2) gradient of 0.93/0.21 atm, measured by the modified e.m.f. technique, are close to 0.4 decreasing in more reducing environments. The variations of partial ionic and electronic conductivities can be described in terms of the oxygen intercalation into the scheelite-type lattice, which results in increasing concentrations of both dominant charge carriers, oxygen interstitials and holes, when p(O-2) increases. Reduction leads to p(O-2)-independent electrical properties, followed by a drastic decrease in the conductivity at oxygen pressures below 10(-15)-10(-9) atm due to a reversible transition into the monoclinic phase. Contrary to the zircon-type CeVO4+delta, no traces of the parent binary oxides were detected in the reduced cerium niobate. (c) 2006 Elsevier B.V. All rights reserved.