Vanadium-aluminum oxide catalysts for propane oxidative dehydrogenation (ODH) are investigated with respect to their physico-chemical properties and catalytic activity as a function of the co-precipitation pH. Modifications of the specific surface area, the particle size distribution, and the chemical nature of the active phase are found. At co-precipitation pH 5.5, an amorphous V-Al-O-x mixed phase was formed by condensation of tetrahedral polymeric [VOx](n)(n-) and amorphous aluminum hydroxide species. At pH between 7.5 and 8.5, the catalyst consists of [VOx](n)(n-) species supported on amorphous aluminum hydroxide. At pH = 10, V2O74- ions are presumably adsorbed on boehmite surface. The propane conversion and the rate of propylene formation per gram-atom of vanadium are higher for the samples prepared at pH greater than or equal to8.5. A correlation exists between the reduction temperature of the vanadium species and the catalytic activity for the propane oxidative dehydrogenation reaction. (C) 2003 Elsevier B.V. All rights reserved.