Utilization of challenging fuels, such as biomass residues and waste-derived fuels, may lead to problems with refractory corrosion. Alkalis and chlorine in such fuels can lead to the formation of low melting deposits and slags on the refractory lining of boilers during combustion. This may cause excessive wear, possibly limiting the service lifetime of the refractories. In this work, the attack of alkali salts to high alumina refractories was studied. The refractory was exposed to four pure salts (K2CO3, K2SO4, Na2CO3, and Na2SO4) and one synthetic ash (K2CO3-KCl). The refractory was exposed to the salts at 700, 800, and 900 degrees C for 168 h. The penetration of potassium into the refractory was determined from horizontal line scan analyses of the sample cross section using scanning electron microscopy with energy-dispersive X-ray analysis. Detailed information on alkali infiltration was obtained by using the Xphase spectral imaging software. Additionally, X-ray diffraction analyses were made on cross sections of the refractory before and after exposure to detect new compounds. Even at the lowest temperature, K2CO3 formed low melting compounds with the refractory, resulting in accelerated deterioration. The main new phases formed were kalsilite, kaliophilite, and leucite. Thermodynamic calculations showed that the initiating step in the corrosion could be the reaction between free silica in the refractory and potassium, even at 700 degrees C. Similar refractory deterioration was caused by the KCl-K2CO3 mixture. Exposure to Na2CO3 led to the formation of nepheline and sodium aluminum silicate. Sodium and potassium sulfate did not show any major corrosive effects; however, at 900 degrees C Na2SO4 melt penetrated deep into the refractory.