Based on experiments on desulfurization, CaSO4 decomposition, and a system approach using theoretical analysis, efficient in-furnace desulfurization in O-2/CO2 combustion was investigated. The influence of combustion conditions and sorbent properties on system desulfurization efficiency was clarified. The global desulfurization efficiency was found to increase with O-2 purity. The global desulfurization efficiency in a dry recycle was higher than that in a wet recycle. The global efficiency of in-furnace desulfurization decreased with initial O-2 concentration. As the temperature increased, the global desulfurization efficiency increased first and then decreased due to the decomposition of CaSO4. In the temperature range investigated, the global desulfurization efficiency in O-2/CO2 coal combustion was much higher than that of conventional coal combustion in air. The global desulfurization efficiency decreased with sorbent size. When the particle radius decreased to one quarter, the global desulfurization efficiency doubled, becoming as high as 80%. The global desulfurization efficiency was very different among the three sorbents investigated, whether in O-2/CO2 combustion or in conventional air combustion. The global desulfurization efficiency increased in the order of Ca(OH)(2), scallop, and limestone in O-2/CO2 combustion, but in the order of scallop, Ca(OH)(2), and limestone in conventional air combustion. Nevertheless, all three sorbents demonstrated much higher desulfurization efficiency in O-2/CO2 combustion than in conventional air combustion.