The inside wall of a coal gasifier is lined with refractory, and the corrosion of the refractory is an important factor affecting the refractory lifetime and the replacement period. This paper examines the changes in microstructure of a chromia refractory due to chemical reactions with slag having varying amounts of Fe2O3. Slag samples were prepared by adding Fe2O3 to KIDECO slag, and static corrosion experiments were carried out at 1550 ℃. The layer of (Fe,Cr)3O4 formation and the depth of Fe depletion in the infiltrating slag were determined. In addition, FactSage equilibrium calculations were carried out in order to determine the conditions of formation, and to compare with the experimental observations. In the sample exposed to KIDECO slag, which has about 10 wt% Fe2O3, the formation of (Fe,Cr)3O4 was not observed. As the Fe2O3 concentration in slag increased, (Fe,Cr)3O4 formation and Fe depletion depth increased. Increasing Fe2O3 concentration also made the slag/refractory interface indistinguishable. Equilibrium calculations predicted that higher Fe2O3 concentrations favor chromite formation at gasification temperatures. The chromite formation was most favorable when the amount of Cr2O3 was limited, as in the case of dissolved Cr2O3 in slag. When the concentration of Fe2O3 in slag was less than 20%, the formation of chromite was least favorable in the system with equal amounts of slag and refractory.