A process is disclosed for producing α-chlor-methylen-triorganylphosphorane carboxylic acid derivatives having the formula (I), in which R stands for C organic substituents; A stands for CN or CO-B; B stands for C or O organic residues that remain inert in chlorinating conditions, by chlorinating phosphoranes having the formula (II) with chlorine. Chlorination is carried out in the presence of a mineral base as hydrogen chloride acceptor, whereas chlorine and the base are synchronously but separately introduced into the reaction mixture according to their consumption. The products (I) obtained by this process represent important intermediates for plant protective substances.