Using gaseous O-18(2), We have investigated the respective role of lattice and adsorbed oxygen in the it-butane oxidation to maleic anhydride (MA) and COx on well-crystallized (VO)(2)P2O7 and on a long term activated VPO catalyst. The reaction was performed at 350 degrees C in a glass reactor, with a dry-ice cold trap and a recirculation pump. The reactor was connected to a mass spectrometer for on-line gas analysis. It has been checked that oxygen isotopic heteroexchange did not occur at a significant rate. The changes with reaction time of the O-18 isotopic labelling shows that only lattice oxygen is active for the formation of MA and the other products. For MA, this conclusion has been further confirmed by comparing the experimental evolution of the isotopic distribution (amu 98, 100, 102 and 104) with calculated mass spectra assuming a progressive replacement of lattice O-16 by O-18 in agreement with a redox mechanism. When the steady-state O-18 labelling of products is achieved, the oxygen balance indicates the presence of O-18 within a few surface layers of the catalyst. This result has been further confirmed by post-reaction analysis using low energy ion spectroscopy, providing a direct evidence of both O-18 and O-16 on the surface.