We present the first evidence of Fe(II) complexation by natural organic ligands in estuarine waters. Across five diverse river/estuary systems we find evidence of terrestrially derived ligands with binding constants (log KFe(II)L) mainly in the range 6-8. These Fe(II) ligands were stable over short time periods (1-2 days), generally equivalent to, or in excess of, ambient freshwater Fe(II) concentrations (which ranged from 12 to 3600 nM) and had similar binding constants to ligands that were leached by water from vegetation and detritus (log KFe(II)L 7-8). A class of terrestrially derived ligands may therefore be important in stabilising Fe(II) concentrations in freshwater systems. However, in coastal seawater the impact of these ligands upon Fe(II) speciation is likely to be diminished due to a combination of dilution, loss of humic material during flocculation and increased ionic strength.The temperate and sub-tropical river systems studied included the Beaulieu (England), Itchen (England), Cape Fear (North Carolina, USA), Winyah Bay (South Carolina, USA) and Loch Etive (Scotland). Freshwaters in each system possessed a broad range of dissolved organic carbon (DOC, 200-1300 μM), labile dissolved Fe (LDFe, Fe < 0.2 μm available to ferrozine after reduction with ascorbic acid, 100 nM-20 μM) and pH (5.5-8.5). In the Itchen estuary, where anthropogenic discharge constitutes > 10% of freshwater input, ligand binding constants were elevated (up to log KFe(II)L 11) and the expected decrease in LDFe with increasing salinity along the estuary was not observed (LDFe and DOC both peaked at a salinity of 7) due to effluent inputs.