The factors influencing the behavior of the CaCO3-H2S reaction were investigated. Three types of limestones, all having high CaCO3 content (>95 % on a mass basis) but differing in petrographic structure. were reacted with H2S in a thermogravimetric analysis system under noncalcining conditions, that is, in the presence of enough CO2 to prevent decomposition of CaCO3. Experiments were carried out over a range of particle size, temperature, and H2S concentration. Pore structure characterization suggested that the particle size decreased during the reaction. The data were analyzed using a shrinking core model, and the effective diffusivity through the product layer was found to vary significantly among the three solids. The obtained results were compared with reactivity data reported for the same stones for the CaCO3-SO2, CaO-SO2, and CaO-H2S reactions in past studies.