U.S. Pat. No. 3,014,958 discloses the reaction of aqueous formaldehyde with methyl propionate, in the molar ratio of from 1:1.5 to 1:20, at 225°-450° C., in the presence of a dehydration catalyst. The reaction is carried out in the presence of not less than 7% by weight of methyl methacrylate, and with addition of methanol. If a catalyst of the general formula K2 O.MgO.Fe2 O3 is employed, a yield of methyl methacrylate of 84%, based on formaldehyde, is achieved, if a molar ratio of methyl propionate:formaldehyde:water:methyl methacrylate of 10:1:1:0.8 is employed and small amounts of methanol are present. If calcium phosphate, calcium sulfate, aluminum phosphate or magnesium phosphate is used as the catalyst, the yields obtained are lower.