A vanadium pyrophosphate oxide (V-P-O) catalyst has been tested for n-butane oxidation under fuel-rich conditions (O-2/C4H10 = 0.6) A rapid decrease in maleic anhydride production was observed with time. XPS analysis showed that this was associated with a rapid reduction of the catalyst surface. From the evolution of the distribution of products, it has been inferred that CO formation mainly occurs by the oxidation of adsorbed butane whereas CO2 is formed by the overoxidation of maleic anhydride. It has been concluded that the n-butane activation occurs on V4+ sites while the insertion of oxygen in the C-4-intermediates leading to MA takes place on V5+ sites. The deactivation in fuel-rich conditions is not so drastic when the V-P-O catalyst has been previously oxidized at a relatively low temperature (500 degrees C). In this case oxygen is stored in the V-P-O surface region, hence favoring the specific O-inserting role of V5+ responsible for IMA formation.