Magnetic iron oxide nanoparticles with a narrow size distribution were synthesized by hydrothermally treating suspensions of iron oxide nanoparticles. Ricinoleic-acid-coated magnetic nanoparticles were co precipitated at room temperature from an aqueous solution of Fe2+/Fe3+ cations by the addition of a base. The presence of the ricinoleic acid on the nanoparticles' surfaces strongly suppressed their growth under the hydrothermal conditions. Because of the strong dependency of the rate of particle growth on their size, the size distribution significantly narrowed during the hydrothermal treatment. The size of the nanoparticles was successfully controlled by the temperature of the synthesis and the amount of ricinoleic acid present in the reaction mixture to between 9 and 30 nm. The presence of the ricinoleic acid on nanoparticles' surfaces enabled the preparation of colloidal suspensions in even moderately polar organic liquids. Measurements of the magnetic properties revealed that the nanoparticles smaller than 14 nm exhibited superparamagnetic behavior and nanoparticles larger than 15 nm displayed single domain ferrimagnetic behavior. The nanoparticles exhibited large values of saturation magnetization of up to 90 emu/g. The strong dependence of the nanoparticles' specific power losses when subjected to an alternating magnetic field on their average size and frequency was demonstrated. The ferrimagnetic nanoparticles showed much higher power losses than the superparamagnetic nanoparticles. (C) 2016 Elsevier B.V. All rights reserved.