Metallic Co nanoparticles of 24-110 nm diameters are prepared by electroless deposition (chemical reduction) in an aqueous solution at room temperature. The reduction process is monitored by an in situ measurement of a mixed potential. The mixed potential, which is above the redox potential of a Co(II)/Co redox pair, drops by the addition of the nucleating agent H2PtCl6 and also decreases with an increase in the H2PtCl6 concentration. Smaller Co nanoparticles are formed. In the smaller particle size, the fraction of the face-centered cubic Co phase increases, and the hexagonal close-packed Co phase decreases. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3265525] All rights reserved.