The effect of the MgO/Al2O3 ratio in an electrofused material obtained with the use of technical alumina and caustic magnesite is investigated. A conventionally thermostable material with minimum range of shrinkage or growth is obtained for the MgO/Al2O3 ratio ranging from 0.38 to 0.48. The alumomagnesium material grows due to the decomposition of the solid solution, the formation of crystals of Al2O3 spinel on the surface, and the interaction between the latter and periclase. The distribution of the elements over the volume of spinel grains studied by x-ray spectral microanalysis confirms the suggested mechanism of growth of the spinel-bearing material upon heating.