Abstract: Acrylic acid or methacrylic acid is prepared by vapor-phase catalytic oxidation of acrolein or methacrolein, with molecular oxygen at a temperature of from 240.degree. to 450.degree. C in the presence of a catalyst represented by the formula:Mo.sub.12 P.sub.a Mg.sub.b Q.sub.b.sub.' R.sub.c T.sub.d O.sub.ewherein Mo is molybdenum, P is phosphorus, Mg magnesium, O is oxygen, Q is at least one of calcium, strontium, and barium, R is at least one of potassium, rubidium, cesium and thallium, T is at least one of vanadium, tungsten and nickel, and a, b, c, d, and e are atomic ratios of the respective elements, where a=0.5-6, b=0.2-6, b+b'= 0.2-6, b' ranges from 0 to a value determined by the magnesium content of the catalyst and the limits of b+b', c=0.2-6, d=0-6, and e is a value dependent upon the oxidation state of the catalyst.