The equilibrium distribution ratio of sulfur between the 16 mass%Cr-14 mass%Ni stainless steel melts and the CaO-SiO₂-MgO-ZrO₂-7 mass%CaF₂ (B(= (mass%CaO)/(mass%SiO₂) = 2.0) slags was measured at 1873 K to clarify the role of ZrO₂ in the desulfurization reaction based on the sulfide capacity concept. The sulfur distribution ratio linearly decreases on increasing the oxygen partial pressure with the slope close to −1⁄2 in the logarithmic scale at a fixed sulfur potential and temperature. The sulfide capacity of the slags is constant up to about (mass%ZrO₂)⁄{(mass%ZrO₂)+(mass%MgO)}=0.3, followed by a linear decrease on increasing the Z⁄(Z+M) ratio. The effect of ZrO₂ substitution for MgO on the decrease in the (apparent) sulfide capacity at Z⁄(Z+M)>0.3 could be explained by the decrease in the basicity of the liquid slag phase due to the formation of CaZrO₃ from the computational estimations for the phase equilibria and the thermodynamic activities of slag components. Based on the X-ray diffraction patterns of the slags, it was confirmed that the peak intensity for the Ca₂SiO₄ is strong through the entire composition range, while the intensity of the peaks corresponding to the CaZrO₃ is relatively strong in the composition of Z⁄(Z+M)=0.42 and 0.64. The sulfide capacity can be expressed as a linear function of the mass fraction of CaZrO₃ in the slags investigated in the present study, while that exhibits a wide distribution at a relatively narrow optical basicity region, resulting from the effect of ZrO₂ on the basicity of the liquid slag phase due to the formation of CaZrO₃.