IRON and manganese oxyhydroxides are constituents of natural aquatic systems and are thought to play a predominant role in the cycling of toxic trace elements in both marine and fresh-water sediments. Because of their dilution in the sediment matrix (typically 0.1–3.0% dry weight) and because no adequate separation technique was available until now, very little is known about their physico-chemical properties and so progress in the modelling of trace-element partitioning and cycling in aquatic sediments has been limited. We have developed a technique which enables the in situ separation and collection of these key constituents by inserting Teflon sheets into lake sediments. The mineralogical composition and surface-adsorption properties of distinct and relatively pure deposits of natural iron and manganese oxyhydroxides can now be studied with conventional instruments and techniques.