Different (VO)(2)P2O7 catalysts have been prepared from the same VO(HPO4). 0.5H(2)O precursor. These catalysts, characterized by XRD, have been compared with respect to their catalytic performances in the n-butane oxidation to maleic anhydride. The surface oxidation state of vanadium has been measured by XPS thanks to the decomposition of the V-2p3/2 peak or the difference of binding energy between the O-1s and V-2p3/2 levels. The relative percentage of V4+ and V5+ depends on the temperature of preparation and on the time of activation under the reaction mixture. The surface (P/V) atomic ratio, in the range 1.5-1.9 as measured by XPS, is not very sensitive to the method of preparation or to thermal treatments. This relatively high (P/V) ratio has been further substantiated by LEIS measurements using He-3(+)/He-4(+) isotopes for quantification. Besides a possible preferential deposit of carbonaceous species on the surface vanadium dimers, it has been proposed that the high (P/V) ratio measured on (VO)(2)P2O7 catalysts could be attributed, at least in parts, to the presence of vanadium vacancies within a few amorphous surface layers.