The effect of CaO on the sulfide capacity of CaO-Al2O3-SiO2 slags was studied from the viewpoint of the ionic structure of alumina in slag. The aluminum coordination number was analyzed using Al-27 500-MHz solid nuclear magnetic resonance spectroscopy and the results were compared with those of the sulfide capacity analysis. The sulfide capacity of slag, in the peralkaline region (XCaO/XAl2O3>1.0), exhibited a linear relationship with respect to basicity (aO2-) as excess free Ca2+ formed a 4-coordinated aluminum unit structure (Al-[IV]; AlO45-) and stabilized the sulfide ions (S2-). However, sulfide capacity in the peraluminous region (XCaO/XAl2O3<1.0) exhibited a nonlinear relationship with respect to basicity (aO2-) owing to the structure of higher-coordinated aluminum units (Al-[V], Al-[VI]; Al3+) and the relative lack of Ca2+. Therefore, the sulfide capacity of high Al2O3-bearing slags strongly depended on the basicity (aO2-) and stability of sulfide ions (S2-), which depended on the competitive behavior of Ca2+ owing to the structural changes in Al2O3. The effect of the aluminum coordination number on the sulfide capacity was discussed in detail using an analysis of the slag structure and thermodynamics model.