Abstract: Methacrylic acid can be prepared with advantage by gas phase oxidation of methacrolein or isobutyraldehyde over Mo- and P-containing catalysts at from 250.degree. to 400.degree. C. with cooling of the hot reaction gas to below 100.degree. C., absorption of the methacrylic acid in a water-operated absorption column, at below 100.degree. C. and partial recycling into the oxidation reactor after admixture of methacrolein or isobutyraldehyde and oxygen by feeding a quantity of methacrolein or isobutyraldehyde equal to the quantity of methacrolein or isobutyraldehyde consumed in the reaction in liquid form together with a polymerization inhibitor into the reactor gas in the lower two-thirds of the absorber column and upstream of the feed point for the fresh methacrolein or isobutyraldehyde or down-stream of the absorption column splitting off the reactor off-gas a side stream which is washed in a wash column operated with water at <10.degree. C.