The physicochemical conversions of spinel-periclase refractories by the action of the penetrating slag melt consist of the ferrous component of the slag being absorbed by the periclase and spinels and being distributed between them in the form of solid solutions while the lime -silicate part of the melt reacts with the periclase and spinels to form meta- and ortho-silicates and alumino- and chromocalcium compounds. The direction of the dominant mass transfer by the action of slags of the system Fe2O3-CaO-SiO2 on the refractory depends on the CaO/SiO2 ratio while the degree of refractory erosion depends to a large extent on the composition and amount of the spinel. The action of acid slag results in refractory material being transferred to the slag so that the refractory-slag boundary shifts in the direction of the refractory. In the action of basic slags on the refractory the dominant process is slag adsorption. In this case the position of the refractory-slag boundary remains almost unchanged. Unlike the solubility of the periclase, that of the spinels increases with the SiO2 content of the slag but its absolute magnitude remains lower than the solubility of the periclase. The composition and percent of the spinels which increase the resistance of periclase refractories to the action of slag depend on the composition of the slag. When a spinel-periclase refractory is to be exposed to the action of ferrous slag of a basic chemical composition, a refractory of periclase and high-alumina spinels is preferred. For acid slags the refractory should consist of periclase and high-chrome spinels containing a maximum (80–100%) of MgCr2O4.