The invention relates to a process for preparing phosphines ##STR1## where, for example R.sup.1 is (C.sub.1 -C.sub.10)alkyl, (C.sub.3 -C.sub.10)cycloalkyl, C(YZ)aryl Y, Z is (C.sub.1 -C.sub.4)alkyl, hydrogen, Aryl is phenyl, naphthyl, anthracenyl, phenanthrenyl, or biphenyl, binaphthylandR.sup.2, R.sup.3 are, independently of one another, phenyl, naphthyl or anthracenyl,by reacting a diarylphosphine ##STR2## where X is bromine, chlorine, fluorine, (C.sub.1 -C.sub.4)alkoxy, O-phenyl, with a reducing agent and a quaternary ammonium compound ##STR3## where A is an anion of an organic or inorganic acid.