In order to determine the 'free' metal ion concentration in aqueous solutions the so-called Wageningen Donnan Membrane Technique (WDMT) has been developed. This involves a continuous flow system in which the donor side and the acceptor side of the WDMT cell are continuously flushed with solution across the membrane. The new cell design allows pseudo equilibrium to be reached for the free metal ions via a Donnan equilibrium across a negatively charged ion-exchange membrane within a reasonable time span. The donor solution contains both 'free' and complexed metal ions. The concentration of the cations in the acceptor solution is either equal to the concentration of the 'free' cation concentration in the donor solution, or it can be calculated using simple correction factors. The optimization experiments have been performed with cadmium and copper, in the presence of various (in)organic complexing agents, at various pH values, and different salt concentrations. In multi-component systems, like the systems used for the experiments with calcium, cadmium, copper, protons and EDTA, the results are in good agreement with the speciation calculations. The transport of the different cations and anions across the membrane can be very well explained by the simplified theory presented. Effects of difference in salt concentration on metal concentrations can be corrected. For the more complicated systems with natural dissolved organic matter (e.g. humic acid) the concentrations measured in the acceptor solution are also in good agreement with the speciation calculations performed for the donor solution.