Effect of ZrSiO4 addition to the binder matrix of the reaction sintered mullite refractories on the alkali corrosion (Na2CO3, at 1370 degrees C) behaviour was investigated. Important improvements on the corrosion behaviour were observed when zircon was introduced to the binder matrix of the mullite refractories. Zircon addition to the mullite forming mixtures improved the densification. Densification effect reduced the tendency of infiltration of the alkali vapours. The investigation of the corrosion product at the reaction interface revealed that after alkali vapour (Na2O) attack, mullite phase decomposed into sodium-aluminate-silicate type phase (carnegieite) and alumina. These expansive phase formation is the main reason for the damage after alkali vapour attack. It was concluded that, apart from densification effect with zircon addition, mullitization and densification behaviour of the binder matrix and type of the stage grains (sinter or fused) are important factors for the alkali corrosion behaviour of the mullite refractories.