The method specified is based on dissolution of a test portion by fusion with a mixture of sodium carbonate, boric acid and sodium nitrate and subsequent acidification with nitric acid, formation of the molybdophosphate complex and extraction with a mixture of ethyl acetate and butyl acetate, followed by selective reduction of the complex to molybdenum blue by means of tin(II) chloride added to the organic phase, spectrometric measurement of the absorbance of the coloured complex at the wavelength of maximum absorption (about 710 nm). Applies to products having total phosphorus contents, expressed as PO4^3-, in the range 0,01 % (m/m) to 1,0 % (m/m).