Straw and other biomasses can cause severe problems when used as fuel in combustion systems. Some of the major problems include high emission of alkalis, HCl, and especially SO2 to the gas phase. The development of low-cost primary measures for achieving a SO2 emission below the European Union (EU) emission limit of 200 mg/Nm(3) without the need for the installation of a flue gas desulfurization unit requires an increased understanding of the chemistry of biomass combustion. For this reason, laboratory combustion experiments with two different high alkaline (HIAL) biomasses doped with additives have been conducted in the temperature range of 800-1100 degrees C. The use of a high-pressure mass spectrometry (HPMS) sampling technique allows the on-line specification of combustion byproducts such as K, Na, S, and Cl species in the combustion flue gases. The results of the measurements show that the SO2 emission may be related to the K/S and K/Si ratios in the initial samples. The release of potassium is mostly dependent on the chlorine content of the samples. Calcium- and potassium-based additives are able to reduce the release of SO2 into the gas phase.