This publication presents recent developments of the knowledge of the vanadium phosphorus oxides used as catalysts for n-butane oxidation to maleic anhydride. We emphasize particularly the nature of the active phase, the active centres and the role of the redox and acido-basic properties contrasting informations between catalysts presenting different degree of disorder. The nature of the active oxygen species and the mechanistic aspects of the reaction are discussed from studies conducted in our laboratory and by comparison with the present knowledge.