4-oxaaldehydes of the formula ##STR1## are prepared by catalytic isomerization of 1,3-dioxanes by a process in which a 1,3-dioxane of the formula ##STR2## where R.sup.1, R.sup.2, R.sup.4 and R.sup.5 are identical or different and are each hydrogen, a straight-chain or branched alkyl, alkenyl or alkynyl radical of not more than 18 carbon atoms, a cycloalkyl or cycloalkenyl radical of 5 to 8 carbon atoms, an aryl, alkylaryl, aralkyl, aralkenyl or alkenylaryl radical of 5 to 16 carbon atoms or a heterocyclic radical and furthermore R.sup.1 and R.sup.2 and/or R.sup.4 and R.sup.5 together with the carbon atoms to which they are bonded may form a cycloalkane, a cycloalkene or a heterocyclic structure, and the stated radicals may furthermore carry substituents which are inert under the reaction conditions, and R.sup.3 is hydrogen or straight-chain or branched alkyl, is isomerized using a metal oxide catalyst, which has been treated with an acid, and/or a silica phase having a zeolite structure.