A 3-(Z)-propenyl cephem compound is selectively prepared by reacting a phosphoranylidene cephem compound with acetaldehyde in the presence of a base in a solvent mixture comprising diethyl ether, formula (I), wherein R is a carboxyl protecting group; R.sup.1 is hydrogen or R.sup.2CH.sub.2CO--; and R.sup.2 is ethyl, 2-thiophenyl, phenyl, p-hydroxyphenyl or phenoxy ##STR00001##
