Abstract: In a method for preparing a phosphorus-vanadium-oxygen catalyst wherein a pentavalent vanadium compound and a trivalent phosphorus compound are brought together in a liquid reaction zone under conditions to provide a substantial amount of tetravalent vanadium and to form a phosphorus-vanadium-oxygen catalyst precursor having a phosphorus to vanadium atom ratio between about 0.9:1 and about 2:1, the improvement of this invention comprises adding a sufficient amount of a surfactant to the liquid reaction zone to form a dispersion which comprises a liquid, the surfactant and a dispersoid of the phosphorus-vanadium-oxygen catalyst precursor.