The speciation of Fe(III) within the aggregates formed in partially hydrolyzed Fe-PO4 solutions was determined by small angle X-ray scattering. The simulation of the scattering curves by models consisting of hard spheres representing the various Fe species shows that the presence of PO4 hinders the polymerization of Fe(III). At a hydrolysis ratio R ([OH]/[Fe]) of 1.0, 60% of Fe are Fe monomers and 40% are edge sharing dimers. At higher R, the aggregates consist essentially of edge-sharing Fe dimers (>84%). The Pe speciation derived from the modeling of the scattering curves confirmed and refined the results obtained on the same samples by extended X-ray absorption at the Fe k-edge spectroscopy in a previous study.