Fe K-edge EXAFS spectroscopy has been used to determine the nucleation mechanisms and to propose a model of colloid and gel formation during the hydrolysis of FeCl3 in the presence of PO4 in acidic solutions. The displacement of OH or O ligands by PO4 anions drastically changes the normal path of Fe(III) hydrolysis in pure water : (i) The Fe(III) nuclei are mainly dimers and linear trimers formed through equatorial edge sharing. (ii) One Cl remains in the first coordination shell of Fe even when gels or precipitates are formed. (iii) Precipitates or gels are obtained through the aggregation of different subunits. In the precipitates, the subunits correspond to three iron dimers associated through one PO4 tetrahedron. The precipitates are formed by aggregating the subunits through a Fe-Fe corner-sharing link (Fe-Fe = 4 Angstrom). In gels, the subunits are two dimers associated through one PO4 tetrahedron. The aggregation of subunits through PO4 leads to the formation of chains. Fe monomer bridges link these chains together by corner sharing, thus forming the gel. The growth mechanisms of Fe-PO4 colloids are dependent on the initial P/Fe ratio.