Preparation and characterization of ultra-stable biocompatible cobalt ferrite-based magnetic fluids has been reported. Synthesized samples have core particle diameter in the range of 4.7 to 14.8 nm, as indicated by TEM. Chemical and crystalline data show that the prepared nanoparticles are cobalt ferrite with a slight deviation from the Fe: Co:: 2: 1 stoichiometry. ATR-FTIR spectroscopy was used to investigate the citrate adsorption onto the nanoparticle surface. The fitted adsorption time-constants were 0.006 and 0.033 min(-1) for nanoparticle diameter of 4.7 and 14.8 nm, respectively. (c) 2005 Elsevier B.V. All rights reserved.