The dissolution behavior of water in binaries, ternaries and high order slag systems in the CaO–MgO–Al2O3–SiO2–FeO slags is thermodynamically modeled in the present study using the hybrid model that is an integration of the capacity model and the modified quasichemical model. The model can well reproduce the dissolution of water both in form of hydroxyl ion with replacing free oxygens in basic slag region and in form of hydroxyl radical with replacing bridged and broken oxygens in acid slag region using surprisingly small number of model parameters.