The relationship between the aluminum–oxygen and sulfur distribution ratio for molten iron and a CaO–Al₂O₃–ZrO₂ slag in equilibrium at 1 873 K was studied using an Al₂O₃ or CaO-stabilized ZrO₂ crucible. The slag and molten iron equilibrated with CaO·2Al₂O₃ and Css (cubic solid solution in CaO–ZrO₂ system) in the Al₂O₃ crucible, and with Ca·OZrO₂ and Css in the ZrO₂ crucible. These equilibrium states coincided with the phase diagram of CaO–Al₂O₃–ZrO₂. From the concentrations of aluminum and oxygen dissolved in molten iron, the Al₂O₃ activity in the slag was determined to be 0.393 in the Al₂O₃ crucible and 0.385 in the ZrO₂ crucible. These activities were compared with the data in the CaO–Al₂O₃ and CaO–ZrO₂ systems. The sulfur distribution ratio was higher a little in the ZrO₂ crucible than in the Al₂O₃ crucible and increased with aluminum content. The sulfide capacity of the fluxes was determined to be 8.77×10⁻⁵ and 9.10×10⁻⁵ in the Al₂O₃ and ZrO₂ crucibles, respectively.