Biomass, especially herbaceous fuel, contains high amounts of volatile alkali metals and chlorine, which may cause problems, such as deposition, corrosion, and agglomeration during combustion. The aim of this work was to obtain more information on the influence of temperature on the transformation of alkali metal species. The combustion experiments with rice straw, coal, and additive FeS2 were performed at various temperatures between 600 and 1,100 degrees C in a laboratory scale reactor. Thermodynamic equilibrium calculations were used to predict the behavior of alkali metal species and interpret the experimental results. The results indicate that the amounts of potassium and sodium in flue gas were quite little compared to those retained in the bottom ash. KCl(g) and NaCl(g), which were the major alkali metal species of flue gas, were independent on a temperature below 800 degrees C. KOH(g) also played an important role in the 1,000-1,100 degrees C interval. In the bottom ash, the fraction of potassium in sulfates decreased from 61.8% at 600 degrees C to 0.5% at 1,100 degrees C. Compared to potassium, most of vaporizable sodium was captured by (alumino-) silicates and retained in the bottom ash.