The concentration of submicron particles, HCl and SO2 in the flue gas from the combustion of straw from wheat, barley and oil seed rape was studied by field measurements on two utility boilers. The concentration of submicron particles varies from 75-2000 mg Nm(-3 ) and the mean diameter in the range from 0.2 to 0.6 mu m. The principal constituents of the submicron particles are potassium sulfhate and chloride. The concentration and composition of the submicron particles depend on the content of K, Cl and S in the straw. These elements are to a great extent volatilized in the combustion zone. A theoretical analysis indicates that the gas to particle conversion occurs during the cooling of the flue gas by the homogeneous nucleation of K2SO4 particles, which act as condensation nuclei for the subsequent condensation of KCl. It is found that the kinetics of the oxidation of SO2, which is slow below 800 degrees C, is important for the chemical changes of the flue gas. The composition of the submicron particles and the concentration of SO2 and HCl can be approximately predicted from the assumption that the flue gas composition is determined by a chemical equilibrium for the sulfation reaction at 812 +/-10 degrees C.