To develop a SO2 sorbent that is highly reactive in the presence of high concentrations of CO2 at high temperature, the effect of the pore-size distribution on the reactivity was investigated at 800 degreesC using a natural limestone, natural lime, modified macroporous lime, and limestone. The modified lime samples were prepared from a kind of natural lime by water-acetic acid swelling and water swelling methods. The modified limestone was prepared from the modified lime by carbonation. Pores smaller than ca. 200 nm in the modified lime and natural lime virtually disappeared, and pores larger than ca. 200 nm also considerably decreased in pore size after carbonization. The reactivity of the modified limestone depended on the degree of development of pores larger than 200 nm, because the macropores provided a diffusion route for SO2 in the sorbent during sulfation.