A process for preparing an antimony/tellurium-containing metal oxide catalyst, comprising preparing a slurry containing an antimony compound, a polyvalent metal compound, and a silica support starting material selected from silica sol and mixtures of silica sol and white carbon, fumed silica or silica hydrogel, heating the slurry at a pH or 7 or less and a temperature of at least 40.degree. C., mixing the above heat treated slurry with a tellurium solution which has been independently prepared by oxidizing metallic tellurium with hydrogen peroxide in the presence of the oxide, oxyacid or oxyacid salt of at least one metal selected from the group consisting of vanadium, molybdenum and tungsten, and then drying and calcining the resulting mixture.