During the gasification of biomass, inorganic species are produced and constitute an important obstacle in this process. The aim of this study is to understand the high temperature behavior of the inorganic species particularly the ashes close to the miscanthus and the straw ashes. A thermodynamic approach is used to obtain this characteristic. The database used for the calculation is first validated by comparison with systems available in the literature and by using an experimental approach. This study reveals the lack of optimization and the lack of data concerning alkali species and particularly K2O-rich compositions. An experimental approach has been used to determine the liquidus temperature of ashes by a classical quenching method. Experiments using sealed capsules largely prevent the volatilization of K2O. The liquidus temperature of three compositions with alkali and one without alkali were measured. A liquid miscibility gap is observed experimentally and by calculation for the straw ash.