The XRD patterns at ambient temperature and at 1500 degrees C showed that the spinel in the Al2O3-MgO castables fired at 1500 degrees C for 3 h has the higher peak intensity, compared to those in Al2O3-spinel castables; the interplanar distance in the set (311) is 2.43 Angstrom for the spinel in Al2O3-MgO castables as well as the spinels in Al2O3-spinel castables using spinels containing 73, 90, and 94 wt% Al2O3, respectively. The corresponding alumina contents of the spinels in these castables were estimated to be around 75 wt%. The smaller grain size of the spinel in Al2O3-MgO castables compared to that in Al2O3-spinel castables is evidenced by the recrystallization of the in situ spinel only occurring in Al2O3-MgO castables as revealed by the XRD patterns at ambient temperature and at 1500 degrees C. The larger amount and smaller grain size of the in situ spinel in the matrix mostly account for the better slag resistance of Al2O3-MgO castables, compared to Al2O3-spinel castables,