When biomass and waste fuels are burned, alkalis and chlorine may form corrosive vapors and deposits, which can cause corrosion of boiler refractories. A laboratory method for studying refractory deterioration at temperatures and conditions relevant for biomass and waste fuel combustion was developed. Alumina refractory samples were exposed to a K2CO3-KC1 mixture at 800 and 1000 degrees C for 168 h. Different approaches to analyze alkali penetration into the refractory material were tested. The infiltration of potassium into the refractory was determined by horizontal and vertical line scan analyses and area analyses of the sample cross section using scanning electron microscopy with energy-dispersive X-ray analysis. A descriptive analysis of alkali infiltration mechanisms was obtained by applying Xphase spectral imaging software. Additionally, X-ray diffraction analyses of the cross section were made before and after exposure to detect new phases. The method gives detailed information on the alkali penetration and the mechanism behind it and can be used to estimate and compare the performance of refractories in various biomass and waste combustion conditions.