Sulfidation experiments at atmospheric pressure were performed in a thermogravimetric analyzer with three uncalcined limestones and a half-calcined dolomite at temperatures between 600 and 850 degrees C and with sorbent particle sizes between 0.4 and 1.6 mm. The effects of reaction temperature, sorbent particle size, gas-phase composition, and H2S concentration were analyzed. For the dolomite, the sulfidation rate increased when the reaction temperature increased and when the particle size decreased. For the limestones, the sulfidation rate did not depend on particle size and the initial reaction rates increased with increasing reaction temperature until 750 degrees C. When the conversion increased, the reaction rate dropped sharply at the highest temperatures, giving a maximum conversion at 650 degrees C for Sastago and Alborge limestones and at 700 degrees C for Omyacarb limestone. Morphological studies with SEM-EDX equipment, BET surface areas measurements, and special tests in the TGA were performed to analyze this behavior. In addition, the effect of the gas composition on the sulfidation reaction was also analyzed with the uncalcined limestone and the half-calcined dolomite and different behaviors were detected. For the dolomite the gas composition (CO2, CO, H-2, and H2O) had no effect using the same H2S + COS partial pressure; however, for the limestones, the sulfidation rate was affected by the COS concentration.