Process for preparing chlorinated ethylenic derivatives of the formula ##STR1## in which R.sub.1 represents acetyl, formyl optionally in the form of an acetal, hydroxy optionally as an ether or ester, alkyloxycarbonyl, alkyl of 1 to 12 carbon atoms substituted by one or more acetyl, formyl optionally in the form of an acetal, hydroxy optionally in the form of an ether or ester, or alkyloxycarbonyl, alkenyl of 2 to 12 carbon atoms containing one or more double bonds optionally substituted by one or more of acetyl, formyl optionally in the form of an acetal, hydroxy optionally in the form of an ether or ester, or alkyloxycarbonyl, or R.sub.1 represents 3-sulpholenyl or a radical of formula ##STR2## in which R.sub.2 denotes a hydrogen or acetyl, by reacting chlorine in a nonpolar aprotic solvent with a compound of general formula ##STR3## in which R.sub.1 is defined as above. The products of formula I, some of which are new, are useful as intermediates in the synthesis of terpene products such as vitamin E.