During oxy-coal combustion process, limestone desulfurization may change from indirect into direct sulfation due to the high partial pressure of CO2. When the wet flue gas is recycled, the water vapor will also be enriched in the furnace and affect the desulfurization reactions. In the paper, two limestone sorbents were used to study the effect of water vapor on direct sulfation. Parameters including water vapor concentration, temperature and SO2 concentration were analyzed. Results show that the presence of water vapor has a negligible effect on the direct sulfation during the kinetically-controlled regime, while enhances the calcium conversion during the diffusion-controlled stage. The presence of water vapor promotes the solid-state diffusion of the sulfated product, and sintering of the product layer is intensified. The vacancies in the particles migrate along crystal grain boundaries and through the crystal lattice, resulting in a reverse flow of mass into the pores. The pore structure character of the products sulfated in the presence of water vapor is improved. (c) 2013 Elsevier Ltd. All rights reserved.