All available thermodynamic and phase-diagram data have been critically evaluated and optimized for the liquid-slag phase and for all solid phases at 1 bar pressure from 298 K to above the liquidus temperatures for the systems MnO-Al2O3 and MnO-Al2O3-SiO2, and a database of model parameters has been prepared. The modified quasichemical model was employed for the molten-slag phase. Calculations using the database were performed with applications to inclusion engineering for Mn/Si killed steel.