The rate and mechanism of H2S reaction with limestone were investigated thermodynamically under pressurized fluidized bed coal gasification conditions to achieve effective desulfurization. :Experimental investigations using a pressurized fixed bed reactor (24 mm I. D. and 300 mm high) were further performed to prove the results obtained by the thermodynamic analysis. The following conclusions were obtained. 1. A new phase diagram for CaCO3, CaO and CaS was proposed for the H2S reaction with limestone using a reciprocal temperature. The diagram took reciprocal temperature, 1/T, as the abscissa, the partial pressure of carbon dioxide, P-CO2, as the ordinate, and the partial pressure ratio of steam to H2S, P-H2O/P-H2S, as the parameter. It was found that P-CO2 was a dominant factor for the equilibrium of H2S reaction with limestone under elevated pressures. Also, P-H2O/P-H2S should be below 500 for the H2S reaction with limestone under pressurized fluidized bed gasification conditions. 2. As the operating pressure increased, which means an increase in P-CO2, the following three reactions were dominantly observed in the rate of HE reaction with limestone: (1) H2S reaction with CaO, (2) CaCO3 decomposition, and (3) H2S reaction with CaCO3. 3. The water shift reaction CO2 + H-2 double left right arrow CO + H2O was catalyzed by CaO and CaCO3. Composition change between CO2 and H2O by the water shift reaction enhanced the H2S reaction with CaCO3, when the composition change reduced the partial pressure ratio, (PCO2PH2O)/P-H2S.