Applicable to products having phosphorus contents of more than 0.0005 % (m/m). Preparation of a solution from a test portion by alkaline fusion either with a mixture of sodium carbonate and borid or of sodium carbonate and sodium tetraborate. Dissolution of the cooled molten mass in nitric acid and adjustment of an appropriate aliquot portion to pH 2. Formation of a phosphomolybdic complex and extraction by 2-methylpropan-1-ol in a sulphuric acid medium. Reduction of the complex by thin(II) chloride in the organic phase and spectrophotometric measurement of the reduced complex at a wavelength of about 730 nm. Has also been approved by the IUPAC.