The reaction of perfluoroalkyl Grignard reagents with phosphorus(III) halides was explored. In the process a new convenient, one-pot, high yield method for the synthesis of (perfluoroalkyl)phosphonic acids has been developed. Perfluoroalkyl Grignard reagents react with phosphorus trichloride or phosphorus tribromide to form (perfluoroalkyl)-phosphonous dihalides. Hydrolysis gives the corresponding (perfluoroalkyl)phosphonous acids. Oxidation of the phosphonous acids with H2O2 produces (perfluoroalkyl)phosphonic acids in 60-78% overall yields, based on the corresponding perfluoroalkyl iodide. The X-ray crystal structures of the toluidinium salts, [MeC6H4NH3](2)[C2F5PO3] and [MeC6H4NH3][C8F17P(O)(2)OH], are reported.