Butane-rich and oxygen-rich mixtures have been fed to a fixed bed reactor loaded with VPO catalyst used in the production of maleic anhydride. The experiments were designed in such a way that the evolution of the state of the catalyst surface and of the catalytic performance along the bed could be followed and related to each other. The results obtained indicate that a marked pattern of catalyst reduction exists in the reactor used under butane-rich conditions, leading to the appearance of V3+ phases. This causes a strong decrease in the selectivity to maleic anhydride.