Speciation of the trace metals Cd, Cu, Pb, Zn and Fe has been determined in the freshwater and estuarine sections of the Yarra River, Melbourne. Samples were taken over a 6-month period. The analytical speciation scheme employed makes use of membrane filtration, chelex resin and dialysis to provide information on particulate, filterable, ion-exchangeable, dialysable and bound metal fractions. Equilibrium computer calculations were used to assist in the interpretation of the species distribution in both sections of the river. Iron was found to exist entirely in bound (non-ion-exchangeable) and particulate forms in both sections of the river. A significant reduction in Fe occurred between the freshwater and estuarine sections due to coagulation of bound forms. The Cd speciation was dominated by ion-exchangeable forms in both river sections (Cd 2+ in freshwater; Cd-chloro species in estuary). The main change between the sections was a loss of bound Cd, presumably due to coagulation of colloidally bound material to which Cd was associated. The filterable Cu was also predominantly in ion-exchangeable forms; calculations suggest Cu-organic complexes dominanted the speciation in the freshwater section and, depending upon the concentration of organic material, either Cu-organics or CuCO 0 3 may predominate in the estuary. There was little change in the filterable Cu between the two sections. The ion exchangeable Pb and Zn levels were also little different between the freshwater and estuarine sections; PbCO 0 3 dominated the Pb speciation and free Zn 2+ that of Zn. Organic complexes appear to be of little importance with these two metals. Both showed a large increase in the particulate metal fraction between the two sections, with little change in either the bound or ion-exchangeable fractions. When data from both river sections were combined, a strong linear relationship was found for Fe, Pb and Zn (and to a lesser extent for Cd) between the total metal and the total bound metal (i.e. particulate+bound). The relationship for Fe is explainable on the basis that almost all the Fe exists only in particulate and bound forms. The Pb and Zn results suggest that, at least in the Yarra system, the increase in total metal levels occur largely in the bound and particulate fractions with little change in the ion-exchangeable fractions. This possibly occurs because these metals are added to the system in bound forms that are not in equilibrium with the aqueous phase.