The preparation of 3-alkoxycarbonyl propenals and 3-dialkoxymethyl propenals of the general formulae Ia and Ib respectively: ##STR1## (R.sup.1 =C.sub.1 -C.sub.3 -alkyl group; R.sup.2 and R.sup.3 =hydrogen, methyl, or ethyl; and R.sup.4 and R.sup.5 =C.sub.1 -C.sub.4 -alkyl groups, which may be joined to form a 5-membered or 6-membered ring), is effected by reacting a corresponding alcohol of the general formula IIa or IIb ##STR2## with oxygen or an oxygen-containing gas in the gas phase in the presence of a metal from Group IB of the Periodic Table or a compound of one such metal acting as catalyst.The target products serve as intermediates for the synthesis of carotenoids.