Surface characterization of V-P-O catalysts used in the oxidation of n-butane and n-pentane to maleic and phthalic anhydrides is reported. Two different states for the same catalyst are selected : after a shea time under reaction conditions (100 h), the ＇non-equilibrated＇ system, and after a much longer time-on-stream (1000 h), the ＇equilibrated＇ catalyst. Characterization has been carried out by following the desorption of molecules with a mass spectrometer after catalysts had been in contact with the hydrocarbons and/or O-2 mixtures at 613 K. Valence state of vanadium under environmental conditions similar to those used in the desorption studies was obtained by XPS. It was concluded that both V-V and V-IV are capable of interacting with the hydrocarbon and that not all the V-IV present at the (VO)(2)P2O7 surface can interact. On the other hand, desorption of molecules resulting from such interaction occurs at lower temperatures from V-IV-rich surfaces(＇equilibrated＇ catalyst), than from the V-V richer surface (＇non-equilibrated＇ catalyst).