Iron oxyhydroxide, especially in its so-called "amorphous" form, plays a key role in the retention and migration of organic and inorganic compounds in soils and aquatic media. The local structure of these "amorphous" species can be directly investigated using synchrotron-based X-ray absorption spectroscopy. In order to study the nucleation mechanisms, FeCl3 solutions were hydrolyzed by NaOH and the precursors obtained at different molar ratios (0 less than or equal to r = (NaOH)/Fe less than or equal to 2.7) were studied by EXAFS. For r greater than or equal to 1.5, Fe polymers formed at equilibrium are hexacoordinated and their local structure is the same as beta-FeOOH. For r = 1.5, the spectra obtained at different aging times show that the starting nuclei are dimers with edge-sharing octahedra. From t similar to 50 min, trimers with edge and corner-sharing octahedra are detected in solution. After 1 h, beta-FeOOH-like polycations, formed by the coalescence of the trimers, can be observed. These polymers are extremely stable because Cl- ions are still incorporated in the structure and are easily displaced by OH-.