The reactions taking place at high temperature during a mold flux slag crystallization are studied in details by means of high temperature powder diffraction. Two heating ramps were performed in two different temperature ranges, showing the formation and decomposition of various phases during the whole crystallization process. A full isothermal kinetic analysis was carried out for the formation of cuspidine, whose results were compared with non-isothermal data provided by DTA analyses, showing a good agreement between the two techniques.