Abstract This study focused on determination of free Ca 2+, Mg 2+, Na + and K + concentrations in a series of CaCl 2 solutions, simulated milk ultrafiltrate and reconstituted skim milk using a recently developed Donnan Membrane Technique (DMT). A calcium ion selective electrode was used to compare the DMT results. The study showed that the free Ca 2+ concentrations measured by the DMT agreed well with calcium electrode data. Concentrations of free Ca 2+, Mg 2+, Na + and K + measured by the DMT agreed with concentrations predicted by existing ion speciation models. It is concluded that the DMT is suitable to measure various free metal ion concentrations simultaneously in complex milk-type systems.