We report here the growth and characterization of iron oxide nanoparticles by polyionic multilayers assembled by electrostatic layer-by-layer adsorption. The polyions poly(diallyldimethylammonium) chloride (PDDA) and polystyrenesulfonate (PSS) were used for the layer-by-layer assembly of the polyions. Nanoparticle nucleation was achieved by a cyclic repetition of Fe oxidative hydrolysis under an inert atmosphere. We employed UV-vis analysis for both monitoring the multilayer assembly process and ascertaining the influence of the Fe oxidative hydrolysis process on the integrity of the polymer film. No degradation of the multilayers due to the particle growth procedure was observed. The nucleation and growth of needle-shaped nanoparticles with a length of about 100 nm and a width of about 10 nm were detected by transmission electron microscopy (TEM). The void volume of the polymer matrix provided an upper limit to the particle size. As shown by selective area electron diffraction (SAED), the polycrystalline particles were identified as beta-FeOOH (akaganeite), a result confirmed by reflection-absorption IR spectroscopy.