Accurate estimates of N and P loads were obtained for four contrasting UK river basins over a complete annual cycle. The fractionation of these loads into dissolved and particulate, and inorganic and organic components allowed a detailed examination of the nutrient load composition and of the factors in¯uencing both the relative and absolute magnitude of these components. The particulate phosphorus (TPP) loads account for 26±75% of the annual total phosphorus (TP) transport and are predominantly inorganic. The inorganic (PIP) and organic (POP) fractions of the TPP loads represent 20±47% and 6±28% of the annual TP transport, respectively. In contrast, the particulate nitrogen loads (TPN) represent 8% or less of the annual total nitrogen (TN) loads and are predominately organic. For dissolved P transport, the dissolved inorganic fraction (DIP) is more important, representing 15±70% of the TP loads, whereas the dissolved organic fraction (DOP) represents only 3±9% of the TP loads. The TN loads are dominated by the dissolved component and more particularly the total oxidized fraction (TON), which is composed of nitrate and nitrite and represents 76±82% of the annual TN transport. The remaining dissolved N species, ammonium (NH 4 -N) and organic N (DON) account for 0 . 3±1 . 2% and 13±16% of the annual TN transport, respectively. The TPN and TPP ¯uxes closely re¯ect the suspended sediment dynamics of the study basins, which are in turn controlled by basin size and morphology. The dissolved inorganic nutrient ¯uxes are in¯uenced by point source inputs to the study basins, especially for P, although the TON ¯ux is primarily in¯uenced by diuse source contributions and the hydrological connectivity between the river and its catchment area. The dissolved organic fractions are closely related to the dissolved organic carbon (DOC) dynamics, which are in turn in¯uenced by land use and basin size. The magnitude of the NH 4 -N fraction was dependent on the proximity of the monitoring station to point source discharges, because of rapid nitri®cation within the water column. However, during storm events, desorption from suspended sediment may be temporarily important. Both the magnitude and relative contribution of the dierent nutrient fractions exhibit signi®cant seasonal variability in response to the hydrological regime, sediment mobilization, the degree of dilution of point source inputs and biological processes.