The modifications of vanadyl pyrophosphate (VPO) during the activation under propane/oxygen/water/nitrogen containing gaseous mixture have been studied and related to catalytic performances of the catalyst in the oxidation of propane to acrylic acid (AA). VPO samples activated at different times starting from VOHPO4.0.5H(2)O have been characterized by using SEM, XRD, (PNMR)-P-31 by spin echo mapping and NH3-TPD techniques. Introduction of water after about 20 h results in enhancing crystallization of the catalyst, reducing surface acidity (also changing acid sites distribution) and promoting the disappearance of V5+-containing phases. At the same time, acrylic acid is produced and its selectivity is improved up to 40 h activation of the catalyst. Stable catalytic performances and physico-chemical properties are then observed. (C) 2004 Elsevier B.V. All rights reserved.