The dependence of iron oxide nanoparticle formation on the structure and thermal properties of Fe oleate complexes has been studied using FTIR, elemental analysis, X-ray photoelectron spectroscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution TEM. The combination of FTIR, elemental analysis, and DSC allowed us to reveal differences between Fe oleate structures for as-synthesized and postsynthesis treated (drying and extraction with polar solvents) compounds. As-synthesized Fe oleate was found to contain a significant fraction of oleic acid, which works as a modifier altering the decomposition process and as an extra stabilizer during iron oxide nanoparticle formation. The thermal treatment of as-synthesized Fe oleate at 70 °C leads to removal of the crystal hydrate water and dissociation of oleic acid dimers, leading to a more thermally stable iron oleate complex whose final decomposition occurs at about 380 °C. Extraction of the as-prepared iron oleate complex with ethanol and acetone results in the removal of oleic acid, leading to complete reorganization of the Fe oleate complex and a dramatic change of its thermal behavior. It mainly leads to an increase in the iron oxide nucleation temperature, thus decreasing the kinetic separation between nucleation and growth processes and affecting the particle size and particle size distribution. When the separation between these processes was about 5 °C, polydisperse nanoparticles were formed, whereas larger temperature separation values allow the formation of monodisperse nanoparticles with sizes in the 8.5−23.4 nm range. The XRD data indicate that iron oxide nanoparticles contain two phases:  wüstite and spinel, with the spinel fraction depending on the conditions of the nanoparticle formation.