A mathematical model was formulated to describe the reaction rate and the particle size distribution of solids in a fluidized bed for the chlorination of rutile by a CO/Cl-2 mixture. The bubble assemblage model was applied to describe the gas behavior in the bed. The particle-size distribution in the bed was determined by a population balance. Predictions of the model gave satisfactory agreement with experimental results. The effects of important variables such as superficial gas velocity, exchange rate between the phases, and the reaction-rate constant were evaluated.