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Magnetite nanoparticles with two different sizes, 5 and 17 nm, have been prepared by the decomposition of organic precursors in an organic media in the presence of oleic acid. The particles were characterized by TEM, X-ray diffraction, and IR and Mössbauer spectroscopy to clarify their structural and physicochemical properties. The samples consist of noninteracting magnetite nanoparticles in both cases with a more uniform size distribution and higher crystal order degree than particles of similar sizes prepared by coprecipitation. A controlled heat treatment of the samples in air leads to the transformation of magnetite to maghemite, which can be followed by the appearance of many IR bands forbidden for a spinel structure. In addition to that, an important reduction of saturation magnetization and coercivity at low temperature takes place whenever the oleic acid is preserved. Finally, Mössbauer spectra at different temperatures clearly show the effect of the nature of the iron oxide phase, the particle size, particle interactions, and structural order at the surface in both magnetite and maghemite as a consequence of oleic coating.