The melting behaviour of a granulated mould powder for the continuous casting of steel was characterised in the temperature range between 350 1300°C by determining the mineralogical constitution. The mould powder as delivered was investigated with simultaneous thermal analysis and heating microscopy. The mineralogical characterization was performed with a reflected light and a scanning electron mikroscope including electron probe microanalysis. The original mould powder mainly consists of wollastonite, fluorite, calcite, glass phases of different chemical compositions, carbon and carbonates and silicates, respectively, rich in sodium. During heat treatment calcination, decarburization and several solid state reactions occured. Cuspidine (Ca3Si2O7CaF2), for example, is formed at 600°C through reaction of fluorite and wollastonite and some glass phases, respectively. Initially it occurs as a seam, but at about 1200°C it shows well-shaped crystals, which melt at 1250°C. At 700°C Na2Ca2Si3O9 is formed and is situated between wollastonite and cuspidine. A connected melt occurs at 1100°C which shows glassy composition. Because of the chosen test conditions carbon did not combust completely at this temperature and therefore its characteristic not-wetting properties regarding the melt were observed. Additionally also the present iron oxides were reduced to metallic iron which occurs finely distributed in the glassy slag at about 1250°C.