AbstractThis paper reports on the application of a chemical fractionation scheme for assessing the amounts, forms and potential mobility (recycling potential) of phosphorus in the sediments of the freshwater Loch Leven, which is currently highly topical in relation to eutrophication and its manifest dense, potentially toxic, blue‐green algae. The findings are placed in the context of the biological importance of the loch, the problems it is experiencing and some management options for its restoration.The fractionation scheme adopted enabled quantification of phosphorus in sediment pore water and in sedimentary solid phases classified as ‘loosely‐bound’, ‘reductant‐soluble’, ‘oxide‐adsorbed’, ‘organic’, ‘apatite‐bound’ and ‘residual’ in the sections of a 16‐cm core collected in January 1990.Vertical profiles of reductant‐soluble and pore water phosphorus and iron show redox‐controlled remobilization and cycling within the sediment to be the principal mechanisms leading to the surface sediment enrichment of phosphorus observed during the winter months.The association of phosphorus with iron oxyhydroxides in the reductant‐soluble fraction, which accounts for 31% of the total phosphorus (2470 mg kg−1) in the 0–1 cm section compared with 0.5% loosely‐bound, 25.6% oxide‐adsorbed, 27.9% organic, 9.9% apatite‐bound and 5.1% residual, restricts the release of phosphorus to the overlying water column in winter.Extrapolation of the data to the whole loch suggests that in early 1990 more than 40 tonnes of phosphorus were associated with the surfaces of iron oxyhydroxides in the upper 10 cm of Loch Leven sediment, about five times that required to raise loch water concentrations to maxima of 0.15 mg L−1observed during the algal blooms of summer under conditions favourable to the release of phosphorus from the sediments.Despite recent measures to reduce external inputs of phosphorus and thereby limit the degree of eutrophication of Loch Leven, there appears to have been little overall change in the inventory of phosphorus associated with iron in the upper 10 cm of sediment since 1970.There should be more detailed seasonal and spatial studies of the chemical fractionation and mobility of phosphorus in Loch Leven sediments to investigate the extent to which release from the reductant‐soluble iron oxyhydroxide fraction (internal loading) contributes to the highly elevate