Nano-octahedral grains of cobalt ferrite (CoFe2O4) with size around 20 nm were synthesized by a hydrothermal route. X-rays and electron diffraction, along with scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy and thermogravimetric analysis were used to characterize the powders. Images and simulations of high-resolution electron microscopy allowed the identification of the shape of the grains. Process parameters such as temperature and time of reaction, reagent concentration, and pH of the reacting medium were optimized. The surfactant cetyltrimethylammonium bromide (CTAB) hindered the formation of goethite, which favored the production of a pure CoFe2O4 powder. The oxidation state of cobalt atoms on the ferrite structure was also influenced by CTAB. The control of the shape of the grains was associated mainly to the nature of the precipitating agent. (C) 2014 Elsevier B.V. All rights reserved.