An ion-exchange column/inductively coupled plasma atomic emission spectroscopy method was employed to measure the thermodynamic stability constant of the 1:1 magnesium-malonate complex. The method involves pumping buffered solutions containing magnesium and malonate ions through a microcolumn of strong-acid-type cation exchange resin until equilibrium between the solution and resin phases is attained. After an air purge and water wash the sorbed magnesium ions, which are present in an amount related to the fi ee magnesium in solution, are eluted with nitric acid and measured by inductively coupled plasma atomic emission spectroscopy. Although nonlinear calibration curves are obtained under the nontrace ion-exchange conditions used, precise and accurate free magnesium concentrations are obtained. The method is straightforward and easy to use, and is applicable to a variety of systems that are not easily measured by other methods.