The bauxite-based and kaolin-based refractory castables investigated were carefully prepared. They are composed of 90wt.% well-graded (coarse, medium, and fine) bauxite or kaolin aggregates, 10wt.% binding matrix and adequate amount of distilled water. The binder mixture was calcium aluminate cement (CAC) containing 80% alumina and magnesium-aluminate spinel (MA-spinel) either preformed or in situ. The castable batches were cast into cubes (25mm×25mm×25mm), left at 100% relative humidity for 24h cured for 7 days under water, and dried at 110°C for 24h. The samples were then subjected to firing at 1550°C for a soaking time of 1h. The microstructure and refractory properties of the prepared castable samples were investigated by using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, permanent linear changes (PLC) test and refractoriness under load (RUL) test. The slag-attack resistance test was applied to determine the degree of penetration of slag and/or the rate of corrosion. The results of these investigations were related as much as possible to each other. The investigated castable samples showed characteristic positive PLC and good volume stability (linear change <1%). They started softening at about 1300°C followed by the rapid subsidence. They showed a somewhat decrease in penetration by the molten slag and a slight increase in corrosion as the spinel (preformed or in situ) content increases. The castable samples containing in situ spinel showed an increase in resistance to slag-penetration and a slightly higher corrosion due to slag attack compared to castable samples containing preformed spinel.