A modified CaCO3 sorbent with an open internal pore structure is prepared and its sulfation characteristics are investigated in an entrained flow reactor at high temperatures (900-1,100°C) and short contact times (20–600 ms) using small particle sizes (< 5 μm). The most distinguishing feature of this modified carbonate (MC) is its 70–75% sulfation conversion within 0.5 s, which is substantially higher than any other sorbents published. The MC is prepared by carbonation-precipitation from a calcium hydroxide suspension by optimizing the operating parameters to generate carbonate particles of the desired pore structural properties. The high initial surface area combined with its open pore structure and pore-size distribution of its calcine contribute to its high reactivity. The calcined MC possesses a significant portion of its pore volume in the 50-200 Å range. This size range represents an optimum pore size for sulfation since it provides a reasonably high surface area and is less susceptible than < 50 Å pore sizes, to pore filling, or pore-mouth plugging due to the formation of higher molar volume CaSO4. Investigation with other carbonates reveals that a much higher portion of their calcines' porosity lies in the smaller pores, which leads to premature termination of sulfation. Results show the impact of internal pore structure on initial reactivity and ultimate sorbent conversion.