Activities of FeO and FeO 1.5 in Na 2 O-SiO 2-FeO-Fe 2 O 3 melts have been investigated in terms of the coordination structure of iron ions. The melts were placed in Pt containers at 1 573 K and equilibrated at partial pressures of oxygen in the range between 10 − 9 atm and 10 − 6 atm, and the activities were derived from Fe concentrations in the Pt containers using the activity coefficient of Fe in Pt-Fe alloys reported as a function of molar fraction of Fe. At the same time, the percentages of Fe 2 + , Fe 3 + in octahedral symmetry (Fe 3 + (oct)) and Fe 3 + in tetrahedral symmetry (Fe 3 + (tetr)) were also measured by Mössbauer spectroscopy. It has been found that the activity coefficients of FeO (γ FeO) are larger than those of FeO 1.5 (γ FeO1.5), suggesting that FeO is prone to liberate from the silicate network more than FeO 1.5. It has also been found that the values of γ FeO monotonically increase with increasing Fe 2 + /Fe total ratio; in contrast, the values of γ FeO1.5 seem relevant to neither Fe 3 + (oct)/Fe total nor Fe 3 + (tetr)/Fe total ratio. The activity coefficients have been discussed from the perspective of the coordination structure via the effective ionic radii of Fe 2 + , Fe 3 + (oct) and Fe 3 + (tetr). The magnitude of effective ionic radii is in the hierarchy of Fe 2 + > Fe 3 + (oct) > Fe 3 + (tetr), and thereby the bond strength between iron ion and oxide ion is in the hierarchy of Fe 3 + (tetr) > Fe 3 + (oct) > Fe 2 +. This suggests that Fe 2 + ions are more loosely bound to the silicate skeleton than Fe 3 + (tetr) and Fe 3 + (oct) ions, which situation would be reflected in the magnitude of the activity coefficients and their dependencies on Fe 2 + /Fe total , Fe 3 + (oct)/Fe total and Fe 3 + (tetr)/Fe total ratios.