The phase partitioning and solubility of Fe as well as its relationship with marine dissolved organic matter (DOM) in different natural seawater and phytoplankton cultures were examined using radiotracer and ultrafiltration techniques to better understand Fe biogeochemical cycling and its biological availability in the ocean. Fe solubility in seawaters was related to the filter's cutoff, with the Fe solubility in the <3 ¿ 103 amu fraction being about one third of that in the <10 ¿ 103 amu filtrate. The Fe solubility decreased from estuarine to coastal to oceanic seawater and then to DOM-free seawater. There was a significant linear relationship between Fe solubility or concentration and the dissolved organic carbon concentration for the seawater of different origins tested, suggesting that Fe solubility was largely controlled by the amount of dissolved organic matter. In addition, Fe solubility was significantly enhanced by the presence of fresh phytoplankton exudates, indicating that the nature of organic ligands also plays an important role in controlling the Fe solubility in seawater. Most of the Fe-bound organic ligands were in the size fraction <10 ¿ 103 amu and decreased from the estuarine to the coastal and then to the oceanic seawater. Among the standard macromolecular organic compounds examined, siderophores (deferoxamine mesylate and ferrichrome) showed the highest binding capacity for Fe, and carrageenan (a high molecular weight--sulfated acid polysaccharide) also slightly increased Fe solubility. Complexation of organic ligands with Fe appeared to be Fe-specific or Fe preferential. Our results highlight quantitatively the importance of DOM in controlling Fe solubility in seawater. Further studies are needed to elucidate the interrelationship between the biogeochemical cycles of Fe and the chemistry of DOM in the ocean.