Potassium/ammonium salts of 12-molybdophosphoric acid were modified by the addition of an Sb5+ salt. The addition of antimony led to a remarkable increase in the thermal structural stability of the compounds obtained. The incipient destruction of the Keggin unit in the mixed potassium/ammonium salt of 12-molybdophosphoric acid was shifted from 400-420 degrees C to 500 degrees C. The potassium salt modified by the addition of one Sb5+ atom per KU decomposed at temperatures higher than 600 degrees C. This property allowed the samples to be used as catalysts for high temperature, gas-phase oxidation reactions, such as the oxidehydrogenation of ethane. The compounds did not undergo structural decomposition at temperatures as high as 540 degrees C under reaction conditions, but were poorly active and selective in ethylene formation. Therefore, antimony-stabilized compounds were further modified by the addition of transition metal ions in order to improve the catalytic performance. The addition of small amounts of iron, chromium and cerium ions led to an improvement of the catalytic performance; the compound was apparently monophasic, and characterized by the cubic crystalline structure typical of the salts of 12-molybdophosphoric acid.