The kinetics of the reaction of CaO powder with SO2 was studied in a differential packed-bed reactor. The reaction rate was controlled by product layer diffusion, except in the initial stage. The intrinsic chemical reaction rate constant was insensitive to the precursor and the calcination history of CaO, but the product layer diffusivity was affected by these two factors in the initial stage and decreased to a constant value with increasing conversion of CaO. The variation of product layer diffasivity with these factors was explained by its dependence on the concentrations of defects in CaO and CaSO4 crystals.