The performance of high-alumina refractories used for aluminium casting significantly impacts the efficiency of metal production. The interfacial reactions with Al-alloys cause corundum and magnesium spinel deposition on the refractory surface, leading to refractory degradation. An experimental study was conducted to investigate the influence of varying barium sulphate (BaSO4) concentrations in a high-alumina refractory on its interfacial reactions with molten Al-alloy in a horizontal tube furnace at 1523 K (1250 degrees C) under inert conditions. This study showed that the Al-alloy reactions with pure BaSO4 would form barium aluminates at the interface. However, in the Al-alloy/refractory system, the interfacial behaviour was strongly influenced by the relative amount of BaSO4, such that up to 5 wt%, the extent of alloy penetration into the refractory increased with increasing BaSO4 contents. Electron-probe micro-analyser and X-ray diffraction studies indicated that the composition of the interface for these refractories was augmented with barium silicates and diminishing anorthite phases. In the presence of 10 wt% BaSO4, the extent of metal penetration into the refractory decreased, whilst for 20% BaSO4, the penetration was higher; these results were attributed to the interfacial presence of celsian (BaAl2Si2O8) and unreacted barium sulphates, respectively. This study suggests that maximising the celsian formation at the interface is critical for optimising the BaSO4 concentration for improving the refractory's performance for Al-casting.