Surface modification of magnetic nanoparticles by organic surfactants is known to provide them with solubility in organic solvents (ferrofluids), which undoubtedly is an important property in several applications and studies. In this report, the main interest is focused on structural, magnetic and adsorption properties of iron oxide nanoparticles that are derived under water/toluene biphase conditions in the presence of oleic acid or oleylamine as the capping agents. The surfactants provide them with excellent stability and solubility in organic solvents like toluene or chloroform. Furthermore, by adding the appropriate surfactant or altering the temperature of the aqueous phase at the initial stage of the reaction we achieve a size control of the nanoparticles within the range 6-18 nm. The presence of capping agents or high reaction temperatures favours the formation of smaller nanoparticles. The adsorption of the surfactants (chemisorption) was identified with FT-IR spectroscopy, while Mossbauer studies have been performed to representative samples in order to identify the presence of either gamma-Fe2O3 or Fe3O4, depending on the reaction temperature. Finally, the magnetic properties of representative samples have been studied at 5 K and room temperature.