The local environment of vanadium in V/Al/O amorphous oxide catalyst has been studied under propane ammoxidation reaction conditions. In situ valence-to-core X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) at the V K-edge showed that acrylonitrile production starts when vanadium in the bulk structure of the oxide material changes its formal oxidation state from +4.8 to +3.8 +/- 0.1. Exposure of the catalyst to pure NH3 at 500 degrees C leads to further reduction of vanadium. Valence-to-core XES has also proved that the level of bulk nitridation of vanadium in the active catalyst was rather small. XPS analysis performed ex situ after the catalytic tests confirmed the presence of significant amounts of N3-, NHx, and -NN-species on the catalyst surface. it can be concluded that activation of V/Al/O catalyst on-stream is mainly associated with reduction of vanadium in the bulk structure of the material and nitridation of vanadium atoms on the surface. (C) 2009 Elsevier Inc. All rights reserved.