The chlorination of hematite was studied by thermogravimetry between 600 and 950 degrees C. The role of convective mass transfer into the boundary layer surrounding the sample, gaseous diffusion into the sample pores, and the effect of the reaction temperature on the reaction rate were analyzed in order to determine the rate-controlling regime. In the 750-950 degrees C temperature range, the reaction rate was significantly affected by diffusion of Cl-2, through the gas film surrounding the sample. In the 600-750 degrees C range a mixed rate-controlling regime with an apparent activation energy of 200 kJ mol(-1) was observed. The diffusion of iron chlorides and oxygen out of the sample pores is proposed as the slowest diffusion step.