Homogeneous CO gas bubble formation in Fe-C-O melts has been studied by assuming that the bubble formation is initiated by the clustering process of CO molecules formed by the reaction of chemisorbed oxygen at the surface of the cluster with carbon atoms impinging on the surface, The electrostatic energy due to dipole-dipole interaction between the chemisorbed oxygen atoms at the surface layer of the cluster was included in the free energy for the bubble formation in a melt system characterized with high surface tension and low vapor pressure, The calculated values of supersaturation for bubble formation, which turn out to be crucially dependent on the oxygen activity, provide the upper bound values of the supersaturation for the CO bubble formation in an iron melt.