An atomic absorption flow detector combined with a gel chromatographic column (Sephadex G-25) gives a sensitive and quantitative method of determining various condensed phosphates such as diphosphate, tri-phosphate, tetraphosphate and Kurrol's salt. This method is based on the automatic recording of atomic absorption at the resonance line of magnesium due to magnesium complexes of condensed phosphates which are produced during the elution of condensed phosphate anions through the column pre-equilibrated with a magnesium chloride solution.