The speciation of phosphorus in Tamar estuary sediment has been determined at eight stations from Gunnislake (riverine end-member) to Torpoint (marine end-member) using the SEDEX (sedimentary extraction) sequential extraction scheme. The seasonal variability of total phosphorus and each of the five fractions is discussed. Total phosphorus, in the range 26a??51 mmol g 1, was stored in the relatively labile fractions, particularly in the iron oxide fraction. The well oxygenated waters of the macrotidal Tamar estuary ensure that the surface sediment layer remains oxic, providing an effective barrier that retains dissolved reactive phosphorus (DRP) within the sedimentary iron oxide fraction and prevents phosphorus migration from the sediment to the water column. The exchangeable and organic phosphorus sedimentary pools, although small, are therefore the main potential sources of phosphorus release to the water column. In particular, this study shows that exchangeable phosphorus and dissolved organic phosphorus (DOP) in the pore water are intimately linked. Finally, the distribution of phosphorus in the estuary is strongly influenced by particle morphology, with most of the sedimentary phosphorus forms exhibiting significant positive correlations with the percent of each grain size.