The aim of this work is to describe the inorganic phase transformation of Miscanthus x giganteus ash at a high temperature taking into account the effect of oxidizing/reducing atmospheres and different harvest periods. A double approach was used: thermodynamic calculations and experimental validation in laboratory devices. The samples were characterized by scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD). Below 800 degrees C, the inorganics are mainly present as salts [KCl, K2SO4, and Ca-3(PO4)(2)], carbonates (CaCO3), and SiO2, while at higher temperatures (>900 degrees C), the solid phase is enriched in CaSiO3 and Ca-3(PO4)(2). The ash starts to melt around 750 degrees C, and the liquid phase is composed of SiO2, K2O, and CaO. The main difference between oxidizing and reducing atmospheres is the form of sulfur. In an oxidizing atmosphere, K2SO4 is formed, while in a reducing atmosphere, only traces of organically attached S were detected by SEM-EDX and most of the S has volatilized. The thermodynamic calculations were performed using the software package FactSage 6.4 with two different oxide databases: FToxid and GToxid. GToxid database predicts a higher solidus temperature (T-s > 900 degrees C) but greater liquid/solid ratio than FToxid (T-s approximate to 740 degrees C). Both databases predict the formation of K silicates or K-Ca (Mg) silicates below 800 degrees C, while the main solid phase observed by XRD was SiO2. Reasons are linked to uncertainties in the initial mass balance of ashes, database uncertainties, and limitations of the diffusion and chemical reaction of elements. The composition of the liquid phase is well-described in both databases, although GToxid gives somewhat closer results to the SEM-EDX analysis with the incorporation of Ca and Mg into the liquid phase.