Sulfation of condensed alkali chloride significantly affects deposition and corrosion in heat transfer surfaces during biomass combustion. This study investigates the heterogeneous sulfation of condensed potassium chloride (KCl) in a fixed-bed reactor at 523-1023 K with levels of 0.1-2% sulfur oxide (SO2), 2.5-10% oxygen (O-2), and 5-15% water vapor. The sulfation rate is determined by measuring the sulfur content in solid residues. No other potassium species are found, and potassium sulfate (K2SO(4)) is the only sulfation product. The KCl sulfation within the temperature range of 523-1023 K was divided into three stages: (1) slow sulfation rate increase below 723 K, (2) slight decline at 723-773 K, and (3) rapid increase over 823 K. The sulfation mechanism transformation occurs between 723 and 823 K. This change in the reaction mechanism is further proven by investigating the effects of SO2 and O-2 reactants and water vapor at 723 and 923 K. The KCl-K2SO4 mixture forms a eutectic to change the reaction mode at high temperatures, thereby explaining the phenomenon, which is further confirmed by the results of scanning electron microscopy. The sulfation reaction rate of condensed KCl is described by the following expressions: dX/dt = 5.3 x 10(-3) exp(-2120/T)(1 - X)(2/3)(C-O2)(0.27)(C-SO2)(0.27)(C-H2O)(0.42) at 523-723 K and dX/dt = 1.20 x 10 (4) exp(-14180/T)(1 - X)(2/3)(C-O2)(0.92)(C-SO2)(0.39)(C-H2O)(0.12) at 823-1023 K.