The sulfidation process of five Ca-based sorbents (three limestones, one dolomite, and one calcium hydroxide) was studied in an entrained bed reactor at high temperatures (900-1100 degrees C). The main variables affecting sorbent sulfidation (Ca/S molar ratio, temperature, H2S concentration, residence time, and sorbent particle size) were analyzed through the knowledge of sulfidation conversion and the evolution of sorbent pore structure under different conditions. A direct relation between chemical structure (limestone-dolomite-hydroxide) and sorbent reactivity was not found. Sulfur retentions up to congruent to 70% were achieved with a limestone, the dolomite, and the calcium hydroxide. However, H2S removal was markedly lower for the other two crystalline-type limestones. The pore structure developed during calcination seems to be responsible for the final capacity of the sorbent to retain the H2S. In this way, the sorbents with a higher porosity and bigger pore sizes showed the highest reactivity and sulfidation capacity at all times and operating conditions. In addition, some sulfidation tests with the sorbents previously calcined in a muffle were made in the drop tube reactor. Complete calcination before sulfidation of the limestones with low porosity increased sulfur removal. However, with the high-porosity sorbents a slight decrease in sulfur removal was obtained.