Unconventional, highly basic slags such as CaO bearing slags are to be used for producing high quality steel. The purpose of this study is to find out what the thermodynamic properties of CaO_satd.-BaO-SiO₂-Fe_tO slags are at steelmaking temperatures. The phosphorus partition between CaO_satd.-BaO-SiO₂-Fe_tO slags and liquid iron was measured as a function of slag composition, together with the CaO solubility. The phosphate capacity of the present system at 1873 K ranged from 10^17.3 to 10^20.0. BaO has been observed to be more effective than CaO for dephosphorization of steel in a thermodynamic sense. The activity coefficients of FeO and FeO_1.5 have been measured and discussed in terms of the stability of orthosilicates. The FeO_n^(2n-3)- anion is likely to be a species of iron in the CaO_satd.-BaO-SiO₂-Fe_tO system, judging from the measurement of Fe³⁺/Fe²⁺ ratio.