The potentiality of species-selective supported liquid membrane device (SLM) for trace metal measurements under natural water conditions has been investigated. Cu(II) speciation studies with various synthetic hydrophilic ligands (sulphosalicylic acid, hydroxyquinoline sulphonic acid and oxalic acid) and natural ligands such as fulvic acids were done using 1,10-didecyl-1,10-diazacrown ether -lauric acid/phenyl hexane-toluene on flat and hollovv fibre supported liquid membranes. The salient features of the results obtained are that only free metal ions are transported across this SLM and the negatively charged complexes and neutral complex do not pass through within the time scale of the measurement. Low free metal concentrations can be measured and most importantly the experimentally observed values of degree of complexation(α_s) are in good agreement with the theoretically calculated ones, the correlation, coefficient being 1.01± 0.02. These findings suggest that SLM can be used as an alternative method for speciation studies in natural waters.