Unsaturated monoesters of the formula (I) ##STR1## are prepared from diesters of the formula (II) ##STR2## where the radicals R.sup.1 to R.sup.5 in the formulae (I) and (II) are identical to each other and each is hydrogen, straight-chain or branched alkyl or alkenyl of from 1 to 10 carbon atoms, cycloalkyl or cycloalkylene of from 5 to 8 carbon atoms, aryl, alkylaryl or aralkyl, by reaction in the presence of a zeolite and/or phosphate as a catalyst with the elimination of a carboxylic acid R.sup.6 COOH where R.sup.6 is hydrogen or alkyl of from 1 to 6 carbon atoms, n being an integer from 1 to 10.