The interaction of oxygen with the surface of vanadyl pyrophosphate has been studied by Temperature-Programmed Desorption (TPD) under vacuum between 300 and 1000 K. It is shown that the adsorption of oxygen onto the catalyst does not occur at temperatures in the range 300-570 K and that it is activated. The behaviour of the main TPD peak around 850 K agrees with second order desorption kinetics showing that adsorption is dissociative. The use of Intermittent TPD (ITPD), which is a particular threshold method, shows that the labile superficial oxygen desorbing up to 900 K is virtually homogeneous with an activation energy of desorption of 327 KJ/mol O-2. Adsorption sites are tentatively assigned to cus V4+, acting as Lewis acid centers, located in the (100) plane of the (VO)(2)P2O7 structure, in the prolongation of the vanadyl columns. The role of labile oxygen in the catalytic oxidation of butane into maleic anhydride is discussed.