The structural properties and oxidation state of vanadium in alumina-supported vanadium-phosphorous oxides at nearly monolayer coverage influence its catalytic performances. The population distribution V-V=O and V-IV=O species in the amorphous VPO phase strongly depends on the sample history. By using alumina-supported VPO instead of conventional bulk catalysts, we facilitate enhanced discrimination between active phase and bulk phase signals in the vanadium-phosphorous oxygen system. This brings light on the speciation of VPO catalysts in general. Our observations were achieved by using mainly in situ EPR & operando Raman-GC spectroscopic techniques at relevant temperatures and gas conditions, showing that the ((VO)-O-iv)(2)P2O7 phase is formed during propane ammoxidation at 753 K. Such transformation is less intense in inert atmosphere at 753 K. Ammonia adsorption on the VPO catalyst surface leads to the formation of VO2+ sites, probably bearing Bronsted and/or Lewis acidity. Propane ammoxidation appears to promote the formation of mixed valence compounds. (c) 2012 Elsevier B.V. All rights reserved.