In order to analyze HCl and SO2 retention in fluidized bed combustors of coal and wastes, chlorination and sulphation of calcined limestone were investigated at 1023 K and atmospheric pressure using thermogravimetry. The rate of chlorination of calcined limestone slightly depended on its particle size and was kept almost constant against the progress of chlorination. In contrast, the rate of sulphation increased with decreasing particle size and steeply decreased with the progress of sulphation as commonly reported. It was found that the sulphation was markedly accelerated in the presence of HCl. Such acceleration of sulphation was remarkable for larger limestone. The level of conversion of CaO to (CaSO4 + CaCl2) always approached 100% in the simultaneous absorption of HCl and SO2. It was observed by SEM that in the chlorination a number of spherical aggregates and large voids were formed on the surface of limestone and that large aggregates with very flat surface and large voids have been formed in the course of the simultaneous chlorination and sulphation. These chlorination behavior and the acceleration of SO2 absorption in the presence of HCl can be due both to the formation of a mobile Cl- ion-containing phase and to the formation of voids playing a role of the diffusion paths for HCl and SO2 toward the interior of a limestone particle. Melting of a eutectic mixture of CaCl2 and CaSO4 might largely contribute to the promotion of SO2 absorption in the case of simultaneous absorption of HCl and SO2.