Metal speciation significantly influences the geochemical cycling of trace metals and can control metal bioavailability and toxicity, Advanced analytical techniques have been applied to address some difficulties encountered in identifying metal organic species. In this method, size-exclusion chromatography (SEC) was directly coupled with inductively coupled plasma mass spectrometry (ICP-MS) to determine conditional stability constants of metal complexes with strong organic ligands in competitive equilibration experiments. molecular size calibration of the column was performed with polystyrenesulfonate standards and the effects of hydrophobic interactions on the column were determined. Metal concentrations corresponding to different organic fractions were measured directly, in the SEC column effluent, by ICP-MS. The method was validated with well-defined organic ligands in ligand competition studies with a single metal and a binary metal mixture. The distribution of metal-ligand species deter mined analytically was shown to correspond to the calculated equilibrium speciation, Model studies using metal complexes,vith weaker organic ligands demonstrated that the technique is subject to kinetic limitations since labile metal-organic complexes can dissociate during chromatographic separation. Nonetheless, the kinetic window of the method should be appropriate for the strong Ligands postulated to control metal speciation in natural waters.