A bench-scale fixed bed reactor was used to study the influences of water vapor and fly ash on Hg-0 removal efficiency over CeO2-modified semi-coke adsorbent (Ce/SC). Adsorption results showed that the mercury removal efficiency of Ce/SC decreased by 30% in the presence of 10% water vapor, and the introduction of 0.5 g fly ash had no significant effect on the Hg-0 removal efficiency of Ce/SC. In the condition of water vapor and fly ash coexisted, the Hg-0 removal efficiency of Ce/SC decreased by only 15%, indicated that the fly ash slowed down the inhibitory effects of water vapor on Hg-0 removal efficiency over Ce/SC, which is mainly due to the interaction between water vapor and Fe2O3 in the fly ash to form Fe-OH groups, furthermore, gamma-Fe2O3 exhibited higher Hg-0 removal performance than alpha-Fe2O3. Hydrogen temperature-programmed reduction (H-2-TPR) revealed that the oxidation activity and capacity of alpha-Fe2O3 and gamma-F2O3 increased significantly after water vapor treatment. X-ray photoelectron spectroscopy (XPS) results showed that the Fe-OH content of alpha-Fe2O3 and gamma-Fe2O3 increased from 37.97% and 15.99% to 44.56% and 43.39%, while the lattice oxygen content decreased from 26.53% and 82.46% to 19.08% and 46.49%, respectively. Density functional theory (DFT) calculations revealed that H2O molecules can be dissociated on both alpha-Fe2O3 (1 0 4) and gamma-Fe2O3 (2 2 0) surfaces to form H atoms and OH fragments, the H atoms bound with O atoms in Fe-O groups of both alpha-Fe2O3 (1 0 4) and gamma-Fe2O3 (2 2 0) surfaces, and the OH fragments associated with the adjacent iron atoms of alpha-Fe2O3 (1 0 4) surface or Fe-O groups of gamma-Fe2O3 (2 2 0) surface to form Fe-OH. The formation of Fe-OH groups increased the oxidation activity and Hg-0 removal efficiency of Fe2O3.