Solid selective adsorption to partition mercury speciation (Hg-0, Hg2+) in flue gas is known as the innovative technology in developing solid agents application in online measurement of mercury speciation by continuous emission monitoring system (Hg-CEMS). Porous materials SiO2, TiO2 and gamma-Al2O3 were chosen as the selective adsorbents and the systematical experiments were conducted in a fixed-bed reactor. Adsorption tests indicated that Hg-0 was scarcely adsorbed by these three porous adsorbents while Hg2+ exhibited an apparent adsorption behavior. gamma-Al2O3 was verified to be efficient in Hg2+/Hg-0 separation because of its high HgCl2 adsorption capacity and SO2 anti-interference ability. The intrinsic mechanisms of HgCl2 selective adsorption and SO2 competition over gamma-Al2O3 surface sites were further investigated. Temperature programmed desorption (TPD) analyses demonstrated that the weak interaction between Hg-0 and gamma-Al2O3 surface was ascribed to a physisorption manner while the adsorption of HgCl2 was characterized chemical bonding. Hydroxyl (Al-OH), Lewis Aln+-Bronsted On- pairs and the coordinatively unsaturated oxygen (O2- ions) were deduced to be effective basic sites for the chemisorption of HgCl2 on gamma-Al2O3. Simultaneously, the competitive adsorption between SO2 and HgCl2 in O2- ions was confirmed to be responsible for the increased breakthrough of HgCl2 over gamma-Al2O3 layer with the existence of SO2.