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Magnetic iron oxide nanopartides in the 10-40 nm size range and with a reduced distribution in size have been synthesized under argon by using ammonium bases R(4)NOH (R = CH(3), C(2)H(5), C(3)H(7)) and a hydrothermal treatment. The size is tuned owing to the base to iron ratio and to the length of the alkyl chain R. We precipitate first ferric hydroxides at pH 1.5-2, then ferrous hydroxide at pH 5.5-6. The rapid increase of pH up to basic pH leads to the formation of magnetic iron oxide particles of 12 nm. For [base] to [Fe] ratio above 3.5, a homogeneous growth occurs during further hydrothermal treatment at 250 degrees C. The higher the quantity of base added and the longer the alkyl chain used, the smaller the particle size produced. For sizes above 20 nm, the Verwey transition at 120 K, characteristic of magnetite, is observed on the field cooling-zero field cooling magnetization curve. The nanoparticles can be described by a core-shell model, that is, a magnetite core surrounded by an oxidized layer close to maghemite. The fractional volume of maghemite increases as the particle size decreases so that below 20 nm, nanoparticles cannot be properly labeled as “magnetite”.