The electrical conductivity of a series of TiO2-supported vanadia-niobia catalysts was measured (i) in pure oxygen to characterize the supported active phases and (ii) in reacting atmospheres to determine their redox properties. In vanadia/titania catalysts, minute amounts of pentavalent V5+ ions were dissolved in the sub-lattice of titania by substitution to Ti4+ sites, the amount increasing proportionally to the loading of vanadia. Similarly, Nb5+ ions also dissolved in the titania sub-lattice, but when niobia were codeposited with vanadia, only V5+ ions dissolved in titania. The variations of the electrical conductivity under oxygen and propane as well as in oxygen-propane mixture showed a redox behavior of the catalysts with the consumption of surface O2-anions during the oxidative dehydrogenation of propane and their replacement by gaseous oxygen. A permanent electron transfer exists from the reduced supported oxidic phase to titania. Niobium sites are not easily reduced and slightly affect the redox properties of vanadium sites which are the active sites, whereas niobium acts as an additive favoring the selectivity to propene.