This paper presents a simple, reliable, and efficient method to produce a stable suspension of monodisperse maghemite (g-Fe2O3) nanoparticles that are highly crystalline and 3 nmto 3.5 nm in diameter. Particles were characterized by X-ray diffraction to determine the crystal phase of the particles and transmission electron microscopy with images used to size the particles. Iron oxide nanoparticles were synthesized through the thermal decomposition of an organic mixture of iron(II) acetate, oleic acid, and 1-octadecene, with particular emphasis on the molar ratio of oleic acid to iron content. These unique conditions produce nanoparticles with physical properties ideal for application as an automotive combustion catalyst in diesel engines and numerous other applications. The one-vessel synthesis method creates an efficient procedure to synthesize iron oxide nanoparticles readily miscible, as made, with Kensol-50H for example, or other petroleum products; producing a ready-made material for mass production with no waste in materials nor subsequent chemical work-up.