The transformation of inorganic constituents in annual biomass was experimentally investigated at grate-combustion conditions. A laboratory fixed-bed reactor was applied to obtain quantitative information of the release of Cl, K, and S to the gas phase from six distinctively different annual biomass fuels. Samples of 4.0 g of biomass were combusted at well-controlled conditions at temperatures from 500 to 1150 degreesC. The elemental release was quantified by analysis of the residual ash and a mass balance on the system. The experimental results revealed that potassium was released to the gas phase in significant amounts at combustion above 700 degreesC. The potassium release increased with the applied combustion temperature for all biomass fuels; however, the quantity released was largely determined by the ash composition. At 1150 degreesC, between 50 and 90% of the total potassium was released to the gas phase. The biomass fuels with an appreciable content of silicate showed the lower release of potassium. Between 25 and 70% of the fuel chlorine was released below 500 degreesC; the residual chlorine was released by evaporation of KCl, mainly between 700 and 800 degreesC. Above 800 degreesC, the fuel chlorine was completely released to the gas phase for all of the samples. Between 30 and 55% of the fuel sulfur was released at 500 degreesC. The samples rich in K and Ca, but low in Si, displayed only a minor increase in the sulfur release as the combustion temperature was further increased. On the contrary, the sulfur release increased abruptly above 700-800 degreesC for the Si-rich samples. On the basis of the release quantification, the overall transformations of the ash-forming elements are discussed at grate-combustion conditions.