The behavior of a chemical element in natural ecosystems depends on the physicochemical form in which the element occurs. In an aquatic system the distribution of the element between the water and the sediment is in many cases strongly dependent on the chemical form in which it occurs in the water phase, i.e. as free anion or cation, in a soluble complex, or incorporated in colloids. The chemical speciation of the element, i.e. its distribution among different physico-chemical forms, often determines its mobility in the system and its uptake in organisms living there. More over different forms of an element may differ strongly in their bioavailability and eco-toxicity. Studies related to trace elements in aquatic ecosystems should therefore preferably be conducted in a way not only registering total levels of the element in different parts of the system but also, if possible, distinguishing between different chemical species in which the element occurs. Most analytical techniques employed in studies of trace elements in aquatic systems determinethe total concentration of the element and not its distribution among different physicochemical forms. In such cases it may be necessary to subject the sample to separation procedures before the element determination. Elements likely to occur in two or more different chemical forms in natural water include chromium, iron, arsenic, selenium, molybdenum, antimony, iodine, mercury, and uranium. The bioavailability and toxicity of such elements may depend strongly on their distribution among different chemical forms. Some examples of speciation studies related to trace elements in aquatic systems will be presented.