Abstract: A process for the manufacture of acylation products of a phosphorous acid having at least two phosphorus atoms in their molecules, which consists essentially of the steps of mixing one mol of phosphorus trichloride with from 2.5 to 3 mols of a mixture of carboxylic acid plus water, said acid being selected from the group consisting of an aliphatic monocarboxylic acid having 2 to 12 carbon atoms and benzoic acid; the share of said water in said mixture being from 1.2 to 1.5 mols; at a temperature up to 80.degree. C; heating the reaction blend thus obtained, after completing of said mixing to 100.degree. C to 160.degree. C and recovering said acylation products.