Process for the preparation of cyclopropane derivatives

In a process for preparing gem-dihalocyclopropane derivatives by reacting an alkene optionally substituted by one or more aromatic groups or halogen atoms with a dihalocarbene in the presence of a catalyst, the improvement which comprises employing as catalyst an onium compound having the general formula ##STR6## in which R.sup.1, R.sup.2 and R.sup.3 each is alkyl or cycloalkyl of up to 40 carbon atoms, Z is an atom of an element of Group VIA of the Periodic Table of the Elements having an atomic number of more than 15 and Y is an anion.