Catalysts for the oxidation of hydrocarbons to acid anhydrides, for example maleic and phthalic anhydrides, are prepared from a vanadium/phosphorus mixed oxide precursor by conditioning it by contact with a strong acid of concentration at least 3 N and solvent extraction to remove unwanted phases. The treated precursor is converted to a catalytically active form by heating. Novel catalysts containing phase X are produced. The catalysts may contain rare earth metal promoters.