The redox properties of vanadium and mixed vanadium and iron have been studied by electrical conductivity measurements. The solids which are active catalysts in the ammoxidation of propane, were shown to be n-type semiconductors with electrons as the main charge carriers. The changes in the electrical conductivity during alternative exposures to propane, oxygen and mixtures of propane and oxygen have been studied at 753 K. The pure vanadium antimonate became a p-type semiconductor after reduction under propane whereas the pure iron antimonate remained an n-type semiconductor. An intermediate behavior was observed for the mixed vanadium and iron compound. The changes in type of electrical conductivity have been explained by a surface transformation with the formation of antimony oxide and a rutile-type solid solution with only cationic vacancies. Such a transformation was limited when iron substitutes vanadium and was absent in pure iron antimonate. (C) 2004 Elsevier B.V. All rights reserved.