(1) On the magesia bed linings of Héroult furnaces, the extent of molten, steel or slag penetration and their corrosion suffered were investigated fundamentally by chemical aaalysis, polararizingmicroscopic examination and x-ray diffraction. (2) In the Iinings, with which common alloy steels were melted, the detrioratlon effect is limited only to the depth. of 10 to 15mm, and fairly amount of iron oxides and limes penetrations are found in this layer. Therefore the periclase (MgO) absorbs much iron oxides, which are contained in solid solution as magnesioferrite (MgO·Fe2O3). Such silicates as monticellite (CaO·MgO·SiO2), meruwinite (3CaO·MgO·2SiO2) and dicalcium silicate (β-2CaO·SiO2) exist in the area between pericase grains. (3) In the Iinings used, for melting of high chromium alloy steels at higher temperature, more severe chaages in quality reach the depth of about 85mm, into which much. chromic oxides, iron oxides and limes penetrate. Thus chromic oxide, iron oxides and periclase react each other and picotite-chromite solid solutions (MgO·Cr2O3·FeO·Cr2O3) are precipitated, on the other hand limes form monticellite in the matrix. (4) Besides, investigations were made on slaglike material heaped on bed lining, magnesia and chromium bricks used for furnace wall. Slaglike material is molten compound of slag, magnesia and chromium bricks, and consisted of magnetite (FeO·Fe2O3), forsterite (2MgO·SiO2), monticellite, diopslde (CaO·MgO·2SiO2), periclase and spinel (MgO·Al2O3) etc. Magnesia bricks are not deteriorated on account of spalling, and consisted by periclase, forsterite and monticellite. Head of chromium brick changes in quality, and its mineral components are spinel, magnetite, forsterite and glass. (5) Finally, mechanisms of corrosion of magnesia lining by slag and molten steel are considered.