Samples of calcined limestone particles having a diameter of about 1 mm were exposed to simulated coal gases containing between 500 and 18 000 ppm H2S for temperatures ranging from 560 to 1100 degrees C in a differential tube reactor. The formation of CaS was followed quantitatively as well as qualitatively to elucidate the reaction mechanism. Contrary to the limited conversion of CaCO3 to CaS, it was found that the limestone particles could be completely converted to CaS by 1% H2S in about 1 h if the particles are precalcined or if the rate of calcination is higher than the rate of sulfidation. The reaction then takes place between CaO and H2S and follows a shrinking-core mechanism. The reaction kinetics is controlled by the diffusion of H2S through the pores of the CaS product layer formed around the lime particle (effective diffusivity between 2.8 x 10(-6) and 5.1 x 10(-6) m(2)/s). The kinetics of the sorption of H2S by CaO is relatively insensitive to the reaction temperature, and the reaction rate does not decrease significantly when the CaO is severely sintered for several hours at 1050 degrees C prior to sulfidation.