The role of various factors affecting the passage of metal ions, ligands, and metal complexes formed with the ligands through ultraflltratlon membranes have been Investigated, in order to select optimal conditions for applying ultraflltratlon techniques for measurement of complexation properties of metal ions and ligands in natural water conditions. The importance of hydration, of electric charge, and of the nature of electrolyte used has been discussed In detail. Factors such as pressure, stirring rate, and the structure of compounds are also briefly discussed.