Abstract: Disclosed is a method for producing methacrolein, which comprises subjecting isobutylene and/or tert-butanol to gas phase catalytic oxidation with a molecular oxygen-containing gas in the presence of an oxide catalyst composition represented by the formula Mo.sub.12 Bi.sub.a Ce.sub.b K.sub.c A.sub.e B.sub.f O.sub.g, wherein A is Co solely, or a mixture of Co and Mg wherein the atomic ratio of Mg to Co is not more than 0.7, B is Rb, Cs or a mixture thereof, and a, b, c, d, e, f and g are, respectively, the specific atomic ratios of Bi, Ce, K, Fe, A, B and O, relative to 12 atoms of Mo.