A tunable synthesis of iron nanoparticles (NIPS) based on the decomposition of {Fe[N(SiMe3)(2)](2)}(2) in the presence of organic superstructures composed of palmitic acid and hexadlecylamine is reported. Control of the size (from 1.5 to 27 nm) and shape (spheres, cubes, or stars) of the NIPS has been achieved. An environment-dependent growth model is proposed on the basis of results obtained for the NP morphology under various conditions and a complete Mossbauer study of the colloid composition at different reacting stages. It involves (i) an anisotropic growth process inside organic superstructures, leading to monocrystalline cubic NIPS, and (ii) isotropic growth outside these superstructures, yielding polycrystalline spherical NPs.