The sulfation of KCl was studied in the gas and molten phase in a laminar entrained-flow reactor. The experiments were performed at 900-1100 degrees C with residence times of 0.24-1.22 s. Small, 65-125 micron particles of KCl were partially vaporized in the reactor and allowed to react with SO2, oxygen, and water vapor. The conversions of KCl to K2SO4 were measured in the products from both the vapor and the molten phase. The sulfation was significantly faster in the vapor phase: up to 100% conversion was obtained in the vapor phase under most conditions but only 0.5-2% conversion in the melt. The sulfation rates both in the vapor and molten phases depend on SO2 and O-2 but not on H2O concentration. The results suggest that SO3 is formed in the gas phase, and that the sulfation rate depends on the availability of SO3. In the gas phase, the sulfation proceeds to the extent that SO3 is available. In the molten phase the sulfation rate was finite even in the presence of excess SO3.