To suppress alkaline elution from steelmaking slag, the dissolution behaviors of mineralogical phases in slag need to be studied. In this paper, primary crystal phases (CaO·SiO 2 , CaO·SiO 2 ·FeO, and CaO·SiO 2 ·2MgO) of CaO–SiO 2 –FeOx and CaO–SiO 2 –MgO systems were synthesized, and their dissolution behaviors and the change in pH were investigated. The results showed that CaO·SiO 2 caused the increase in pH by the dissolution of alkali. Therefore, the alkali elution from steelmaking slag is caused by not only free CaO and 2CaO·SiO 2 but also CaO·SiO 2 . Then, slags corresponding to actual steelmaking composition in the system CaO–SiO 2 –FeOx and CaO–SiO 2 –MgO systems were synthesized and their dissolution behaviors were observed. The result showed that the pH values after the dissolution of every slag were lower than that of CaCO 3 , even though water soluble phases were contained in slag. Therefore, alkali elution from slag containing soluble phases can be suppressed when the mass fractions of these phases in slag are low enough.