By applying the chemical equilibrium method, the equilibrium between liquid iron and 2CaO∙SiO₂-3CaO∙P₂O₅ solid solution has been measured with oxygen partial pressure of 5.22×10⁻¹² atm at 1823 K and 1.41×10⁻¹¹ atm or 4.25×10⁻¹⁰ atm at 1873 K. The phosphorus partition ratio between 2CaO∙SiO₂-3CaO∙P₂O₅ solid solution and liquid iron was observed. The activity of P₂O₅ relative to hypothetical pure liquid P₂O₅ was determined from the phosphorus content in liquid iron and the reported thermodynamic data. The activity of P₂O₅ in 2CaO∙SiO₂-3CaO∙P₂O₅ solid solution increased with the increase of 3CaO∙P₂O₅ content in solid solution. The activity coefficient of P₂O₅ in 2CaO∙SiO₂-3CaO∙P₂O₅ solid solution also increased with the increase of 3CaO∙P₂O₅ content in solid solution.