Isobutane was catalytically oxidized to methacrylic acid with molecular oxygen above 300 degrees C over crystalline particles of pyridinium 12-molybdophosphates which were activated by heat-treatment under a nitrogen stream up to 420 degrees C. During the treatment two third of pyridine were released from the sample and one third remained accompanied by the formation of orthorhombic structure. It was proved by IR studies that the catalyst became a reduced state by this treatment. The reduced state was highly stable even under catalytic oxidation conditions with molecular oxygen. 50% selectivity to methacrylic acid and 22% to acetic acid were achieved at 22% conversion of isobutane. The highly stable reduced state and structure of the activated heteropolymolybdophosphate catalyst seem to be responsible for the high activity for isobutane oxidation and high selectivity to methacrylic acid formation.