The stability of the catalyst is a key issue in the operation of a circulating fluidized bed reactor (CFB) for obtaining maleic anhydride. To study the catalyst degradation process, two VPO samples have been compared: one before operation ("fresh") and the other ("used") after operation in a CFB. Since the operation in the CFB implies the catalyst reduction in one of the reactors and its oxidation in the other, transient experiments have been chosen as a suitable method for comparing the performance of both catalysts. in order to assess the possible modifications of properties, characterization was performed by various techniques (XPS, XRD. SEM, BET. ICP and TPR). The results show a decrease in performance of the used catalyst, with lower activity and yield and a slower oxygen transfer rate, which probably are related with morphological and structural changes such as the lower surface area, the larger size of the crystals, and the appearance of a new phase. In addition, the deposition of iron during the operation could also affect the catalyst performance. (C) 2008 Elsevier B.V. All rights reserved.