The concentration change of iron oxide in mould fluxes has been measured to elucidate its formation kinetics in relation to continuous casting. Three physical and chemical conditions were changed in experiments, namely oxygen partial pressures in atmosphere, SiO2 activities in mould flux and oxygen contents in molten iron. Pre-melted mould fluxes were held in contact with molten electrolytic iron or Al-killed iron in an alumina crucibles at 1891 K in an argon or air atmosphere for different holding times. The concentration of iron oxide in mould flux was determined by X-ray fluorescence analysis. In case of electrolytic iron, iron oxide concentrations increased with holding times and reached ca 3 mass% within 1.2 ks, independently of ambient gases. This independence suggests that the oxygen source was not the atmosphere. Higher SiO2 activities resulted in lower concentrations of iron oxide, which suggests that SiO2 would not contribute to oxidation of molten iron. In case of Al-killed iron, iron oxide concentrations at maximum decreased to ca 0.5 mass% possibly due to lower oxygen contents in molten iron. This decrease suggests that oxygen source would be oxygen contained in molten electrolytic iron. Another finding about the iron oxide profile in mould fluxes suggests that mass transfer of iron oxide in the boundary layer is the rate controlling step, which is supported by the fact that a reasonable value of mass transfer coefficient k (reaction rate constant) of 4 × 10-6 cm·s-1 has been derived on the basis of the concentration change of iron oxide with time.