Oxidation of n-pentane on bismuth-doped vanadyl pyrophosphate was shown to give three anhydrides : maleic, phthalic and citraconic. The presence of a bismuth additive (Bi/V = 0.1) increases the selectivity to citraconic anhydride which is formed via skeletal isomerization of n-pentane to isopentane, followed by its oxidation. Phthalic anhydride seems to be formed via condensation of maleic anhydride with intermediate C-4 olefins.