Iron oxides in mould fluxes enhance heat extraction from the molten steel to the mould due to energy absorption by d-d transitions of Fe2+ and re-emission. Thus, the existence of iron oxides is against mild cooling of molten steel. In this study, mould flux powders containing ca 2 mass% Fe2O3 were mixed with sufficient amounts of CaSi2. The mixtures were contained in alumina crucibles and melted at 1673 K in Ar-H2 atmosphere. The melts were poured into brass moulds to obtain glassy samples 5 mm thick. The Fe2O3 concentration was analysed by a scanning electron microscope with an energy dispersive spectrometer. The concentration decreased from ca 1.71 mass% to 0.49 mass% within 5 min and then settled down. Mass transfer of Fe2O3 is supposed to be the rate-controlling step at high temperature. The mass transfer coefficient has been calculated to be 1.8×10-3 cm・s-1, which seems reasonable. Since crystallisation of mould flux enhance heat reflection from the molten steel to reduce heat transfer, crystallisation kinetics of mould fluxes has been investigated using the Avrami equation, which suggests that additions of reducing agents such as CaSi2 suppress the crystallisation process. In addition, CaSi2 additions result in a dramatic decrease in the total radiative heat flux across mould fluxes in glassy state and would be effective for mild cooling of molten steel.