Cobalt ferrite (Co Fe2 O4) possesses excellent chemical stability, good mechanical hardness, and a large positive first order crystalline anisotropy constant, making it a promising candidate for magneto-optical recording media. In addition to precise control of the composition and structure of Co Fe2 O4, its practical application will require the capability to control particle size at the nanoscale. The results of a synthesis approach in which size control is achieved by modifying the oversaturation conditions during ferrite formation in water through a modified coprecipitation approach are reported. X-ray diffraction, transmission electron microscopy (TEM) diffraction, and TEM energy-dispersive x-ray spectroscopy analyses confirmed the formation of the nanoscale cobalt ferrite. M-H measurements verified the strong influence of synthesis conditions on crystal size and hence, on the magnetic properties of ferrite nanocrystals. The room-temperature coercivity values increased from 460 up to 4626 Oe under optimum synthesis conditions determined from a 23 factorial design. © 2008 American Institute of Physics.