The sulfation of gaseous KCl by H2SO4 in the presence of O2 and H2O was investigated in a laboratory laminar flow reactor at atmospheric pressure and temperatures ranging from 873 to 1523 K. The degree of sulfation was characterized by analysis of the elemental composition (Cl, S, and K) of the submicrometer particles captured on a filter downstream of the reactor. Furthermore, concentrations of HCl and SO2 in the outlet were measured. The experimental results were interpreted in terms of a detailed chemical kinetic model for the K/S/Cl chemistry. The degree of KCl sulfation depends on the reaction temperature and the reactant concentrations. The results show that H2SO4 is effective in sulfating KCl to K2SO4 at temperatures in the range 1073–1273 K, in particular at high H2SO4/KCl ratios. At temperatures of 1373 K and above, the sulfation efficiency decreases rapidly because alkali sulfate formation is no longer thermodynamically favored. The kinetic analysis shows that reaction is initiated by decomposition of sulfuric acid: H2SO4 + H2O ⇄ SO3 + H2O + H2O. Then KCl reacts directly with SO3 to form the intermediate KSO3Cl, which is further converted via KHSO4 to gaseous K2SO4. As the gas is cooled downstream of the isothermal zone, potassium sulfate nucleates homogeneously to form an aerosol, promoting further sulfation in the gas phase. The simulations are in satisfactory agreement with the experimental results, predicting correctly the temperature window for the process. However, the model overpredicts the SO2 yield at high temperature, indicating that the K/S interactions under these conditions are not fully understood. The results are important, both for validation of models of K/Cl/S interactions in combustion and for characterization of the sulfation of KCl by sulfuric acid to evaluate the potential of the sulfur recirculation process and identify the optimum process conditions.