Pure ceria and two surface-phosphated ceria containing different amounts of phosphorus, 1.1P/CeO2 and 2.2P/CeO2, catalysts for isobutane oxydehydrogenation, were characterized by in situ electrical conductivity measurements. Their electrical conductivity was studied as a function of temperature and oxygen partial pressure and was followed with time during sequential exposures to air, nitrogen, isobutane-air mixture (reaction mixture) and isobutane-nitrogen mixture in conditions similar to those of catalysis. All the materials appeared to be n-type semiconductors under air with quasi-free electrons as the main charge carriers and their electrical conductivity increased following the order: CeO2<1.1P/CeO2<2.2P/CeO2. While CeO2 and 1.1P/CeO2 remained n-type semiconductors when contacted with isobutane-nitrogen or isobutane-air mixture, 2.2P/CeO2 became a p-type semiconductor with positive holes as the main charge carriers. For 2.2P/CeO2 catalyst the oxydehydrogenation reaction involves surface lattice O- species, whereas for CeO2 surface lattice O2- anions are involved. For 1.1P/CeO2 catalyst the reaction mechanism involves both surface lattice O2- anions and surface lattice O- species. These results explained the difference in catalytic performances encountered on these solids. In all cases the overall reaction mechanism can be assimilated to a Mars and van Krevelen mechanism. (C) 2012 Elsevier B.V. All rights reserved.