When applied for desulfurization in coal combustion process, common Ca-based sorbents have very low Ca conversion and some additives were found can improve the sulfation effect of sorbents greatly. The sulfation characteristics and pore structure of Ca-based sorbent modified by some inorganic compound were investigated in this paper. Although the sulfur capture capability of M-CaO (calcined from modified limestone) greatly exceeds that of N-CaO (calcined from original limestone), the M-CaO sulfation phenomena cannot be explained well based only on pore structure, as has been suggested by some investigators. To find the sulfation mechanism, the crystal structure of M-CaO was measured. Based on experiments and solid-state ions diffusion theory, a new sulfation mechanism is proposed: lattice defects in M-CaO lead to higher calcium conversion. Based on this conclusion, a rule that estimates whether or not a compound can act as a Ca-based sorbent additive is suggested. (C) 2002 Published by Elsevier Science B.V.