The reactivity of equilibrated VPO catalysts with different P/V ratio was investigated under steady and unsteady conditions, with the aim of gaining information on the status of the active surface in reaction conditions. Specific treatments were applied, and then the unsteady performance was followed along with the reaction time, until the steady behaviour was recovered. These transient states made possible to infer that the nature of the true active surface is a function of the VPO characteristics and of the reaction conditions. With the catalyst having P/V ratio >1.0, the hydrolysis of the VPP surface under working conditions at temperatures higher than 340-350 degrees C, followed by the oxidation of vanadium, generates an active layer made of vanadium oxide and of P2O5/H2O. When instead the P/V ratio is close to 1.0, the VPP surface is oxidized to VOPO4 already at mild temperature under working conditions, with development of an active layer which is very active but unselective to maleic anhydride at 380 degrees C. Higher temperatures then lead to the hydrolysis of VOPO4, with generation of an active layer which is similar, and behaves similarly, to that one which develops with the catalyst having P/V ratio >1.0. (C) 2006 Elsevier B.V. All rights reserved.