CaS formed from the CaO sorbent during desulfurization in coal gasifiers has to be converted to CaSO4 before disposal. CaS is mainly decomposed to CaO and SO2 by O-2 and then CaO is converted to CaSO4 by SO2 and O. The role of H2O in the oxidative decomposition of CaS with O-2 was studied using reagent grade CaS and (H2O)-O-18. The following results were obtained: (1) there is a synergistic effect of H2O and O-2 on the oxidative decomposition of CaS to CaO and SO2; (2) H2O reacts with CaS to form CaO, SO2 and H-2 in the absence of O-2; (3) the oxidative decomposition of CaS to CaO and SO2 Occurs stepwise; (4) H2O directly reacts with CaS in the presence of O-2; (5) H2O plays an important role in the oxidative decomposition of CaS even if the O-2 concentration is high. (C) 2003 Elsevier Ltd. All rights reserved.