A process for making a catalyst containing oxides of molybdenum, bismuth, iron, cesium and, optionally, other metals, such as tungsten, cobalt, nickel, antimony, magnesium, zinc, phosphorus, potassium, rubidium, thallium, manganese, barium, chromium, boron, sulfur, silicon, aluminum, titanium, cerium, tellurium, tin, vanadium, zirconium, lead, cadmium, copper and niobium wherein metal compounds are dissolved and then precipitated as a catalyst precursor which is calcined to form a mixed metal oxide catalyst. The process of the present invention uses an organic acid, such as acetic acid, instead of nitric acid to dissolve the bismuth compound and, optionally, other metal compounds. The catalyst synthesized by this process may be used for the production of unsaturated aldehydes, such as methacrolein, by gas phase catalytic oxidation of olefins, such as isobutylene.