Reported in this paper are the preparation and properties of epsilon-Co nanocrystals coated by poly(ethylene glycol)-block-poly(acrylic acid) (PEG-b-PAA). These particles were prepared via the thermal decomposition of Co-2(CO)(8) at 185 degrees C in 1,2-dichlorobenzene, in the presence of the surfactant PEG-b-PAA and the co-surfactant trioctylphosphine oxide. At a given initial Co-2(CO)(8) concentration, the size of the particles increased with increasing Co-2(CO)(8)-to-PEG-b-PAA molar ratio, and could be tuned between similar to 5 and similar to 20 nm. The size distribution of the particles narrowed as the Co-2(CO)(8) concentrations increased. The resultant particles were dispersible in a wide range of solvents, including chloroform, N,N-dimethylforamide, and water, which solubilized PEG. Magnetic measurements revealed that the particles possessed saturation magnetization close to that of bulk Co, suggesting high purity of the particles. (C) 2010 Elsevier Ltd. All rights reserved.