The velocity of formation of gaseous chlorine which results from the reaction of NaCl or CaCl2 with gas mixtures of SO2-O2 or SO2-O2-SO3 has been measured. For mixtures of SO2-O2 the reaction rate depends on the composition of the gas atmosphere at constant temperature. For concentrations of SO3 above one percent, the rate velocity is determined only by the SO3 concentration. Furthermore, the influence of admixtures of Fe2O3 on the kinetics of these reactions has been examined. The complex sequence of reactions has been subdivided into partial reactions in order to investigate the various processes occurring during chloridizing roasting, most of which are largely unknown as yet. In the present study, the reactions of the NaCl and CaCl2 chlorine vehicles with SO2-O2 gas mixtures have been examined in the temperature range of between 500 and 600C, first with mixtures of changing composition and then with increased SO3 content by measuring weight increases. Furthermore, the effect of admixtures of Fe2O3 on these reactions, and the behavior of pure Fe2O3 with the reaction gases has been examined. Kinetic tests determine the speeds with which these reactions occur.