A derivative neutron activation procedure determines the element magnesium using the chelating agent 5, 7-dibromo-8-hydroxyquinoline. By selective solvent extraction, the magnesium chelate is isolated from interfering elements and separated from excess chelating agent using paper chromatography. The amount of magnesium present is then inferred from the bromine activity after neutron irradiation of the isolated chelate. The method improves the sensitivity for magnesium detection by a factor of 5 and removes the time constraint characteristic of conventional activation determination of magnesium.