Carbonyl compounds of the formula where R1 is a hydrogen atom or an alkyl radical having from 1 to 3 carbon atoms, R2 is a hydrogen atom, an unsubstituted or C1-C3-alkyl-monosubstituted to -trisubstituted C2-C4-alkenyl radical or a radical of the formula where R3 is a hydrogen atom or together with R4 is an oxygen atom, R4 is the radical OR6 or together with R3 is an oxygen atom, R5 is a hydrogen atom, an alkyl radical having from 1 to 8 carbon atoms, an unsubstituted or C1-C3-alkyl-monosubstituted to -trisubstituted C2-C4-alkenyl radical or a cyclohexyl or cyclopentyl radical and R6 is an alkyl radical having from 1 to 4 carbon atoms, a cyclohexyl or cyclopentyl radical or a radical of the formula —CH2—CHO or —CH2—CH2—O—CH2—CHO, are prepared by gas-phase oxidation of methanol or alcohols of the formula where R1 and R5 are as defined above, by means of an oxygen-containing gas in the presence of copper- and/or silver-containing catalysts, the catalyst system used comprising at least one phosphorus-containing copper- and/or silver-containing catalyst.