The infiltrations of synthetic coal slag into 99%Al2O3, 85%Al2O3–15%SiO2, and 90%Cr2O3–10%Al2O3 refractories with a temperature gradient induced along the penetration direction were compared to one another. The infiltrating slag was synthesized with a composition that is representative of an average of the ash contents from U S coal feedstock. Experiments were conducted with a hot-face temperature of 1450°C in a CO/CO2 atmosphere. Minimal penetration was observed in the 90%Cr2O3–10%Al2O3 material because interactions between the refractory and the slag produced a protective layer of FeCr2O4, which impeded slag flow into the bulk of the refractory. After 5 h, the 99%Al2O3 sample exhibited an average penetration of 12.7 mm whereas the 85%Al2O3–15%SiO2 sample showed 3.8 mm. Slag infiltrated into the 99%Al2O3 and 85%Al2O3–15%SiO2 refractory systems by dissolving the respective refractories' matrix materials, which consist of fine Al2O3 particles and an amorphous alumino-silicate phase. Due to enrichment in SiO2, a network-former, infiltration into the 85%Al2O3–15%SiO2 system yielded a higher viscosity slag and hence, a shallower penetration depth. The results suggest that slag infiltration can be limited by interactions with the refractory through the formation of either a solid layer that physically impedes fluid flow or a more viscous slag that retards infiltration.