Permeation liquid membrane (PLM) based on liquid-liquid extraction principles is an emerging analytical tool for making in situ trace metal speciation measurements. A PLM comprising didecyl 1, 10 diaza crown etherlauric acid in phenylhexane/toluene has been developed for measuring free metal ions (e.g. Cu, Pb, Cd and Zn) concentration under natural water conditions. The capability of PLM for making speciation studies has been demonstrated using synthetic and natural ligands. Application of in situ preconcentration of trace metals in diverse waters using specially designed hollow fibre PLM are reported.