The wetting behavior of single crystalline MgO by fayalite-type slag melts has been studied by the sessile drop technique incorporated with high temperature X-ray radiography. The experiments were performed under oxygen partial pressures of 10 -6 and 10 -4 Pa at 1473 K. Mass ratios of iron to silica, Fe/SiO 2, in the slag employed were 1.4 and 2.0. To investigate the interfacial reaction, experiments using high density MgO crucibles were also conducted. The reactions at the interfaces were examined by Electron Probe Microanalysis. The contact angles between the slag melts and the MgO substrates were found to be relatively low, ranging from 30 to 10 degrees. The surface tension of the slag was 0.4 N/m for a slag of Fe/SiO 2 = 2.0. Two phases, magnesiowustite and olivine, were observed at the interface. Interdiffusivity in magnesiowustite solid solution was determined as a function of iron oxide content. Phase relations between magnesiowustite and olivine, and the growth rates of both phases, are also discussed.