CaS formed from the CaO sorbent during desulfurization in coal gasifiers must be converted to CaSO4 before disposal. CaS is mainly decomposed to CaO and SO2, and then the CaO is converted to CaSO4 by SO2 in the presence of H2O and O-2. However, the inner portion of the CaS particles cannot be converted to CaO and CaSO4, because the pores are plugged and oxidative gases (H2O and O-2) cannot come into contact with the interior of the CaS particles. In this study, the effect of the pore-size distribution of the CaO sorbent on the sulfurization and consecutive oxidative decomposition of the formed CaS in the presence of H2O at high temperature have been investigated, using three samples with different pore structures but similar surface areas. The following results have been obtained: (i) the macroporous CaS can be easily converted, because the oxidative decomposition of the macroporous CaS occurs without pore plugging; (ii) pores with a size of > 100 nm in the sorbent can have an important role during the sulfurization and consecutive decomposition of the formed CaS; and (iii) the molar CaO/CaSO4 ratio in the decomposed CaS sample is affected by the pore structure.