In about 160 experiments, +6 mesh (1-2 mm diameter) limestone samples were exposed to an atmosphere of CO2 (87-88%), H-2 (1-2%), CO (4-5%), H2O (4-5%), and H2S (0.5-1.85%) for various temperatures (570-860 degrees C) and durations (5-240 min). Limestone sulfidation was followed quantitatively as well as qualitatively to elucidate the reaction mechanism. The morphology of the inside and of the outside of reacted limestone samples was observed with a scanning electron microscope, and X-ray maps of the sulfur content of the reacted samples were generated. The reaction between H2S and limestone is first-order with respect to the H2S partial pressure in the 560-660 degrees C range, where the kinetics is controlled by the chemical reaction with an activation energy of about 39 kcal/mol. Between 660 and 710 degrees C the reaction rate decreases as the temperature increases and the apparent order of the reaction changes from first- to half-order. Finally, in the 710-860 degrees C range, the reaction becomes controlled by solid-state diffusion through the CaS product layer with an activation energy of 30-40 kcal/mol.