Electrochemical nitriding of Co was achieved by potentiostatic electrolysis at 0.80 and 1.20 V (vs. Li+/Li) in LiCl-KCl eutectic melt containing Li3N at 723 K. From X-ray photoelectron spectroscopy and X-ray diffraction analyses, it was confirmed that the obtained nitrided layer consisted of Co3N; the free energies of formation of this nitride is positive and the equilibrium nitrogen partial pressure of those are of the order of 10(10) atm at 723 K. This result suggests an equivalent nitrogen pressure of at least the order of 10(10) atm was imposed by the adsorbed nitrogen atoms (N-ad) formed by anodic oxidation of nitride ion (N3-) at Co electrode surface. (C) 2004 The Electrochemical Society.