This study investigated the single and dual adsorption of vapor-phase Hg0 and/or HgCl2 using an innovative composite sulfurized activated carbon prepared by combining two sulfur impregnation methods. Thermogravimetric analysis (TGA) technology was applied to determine the saturated adsorptive capacity of Hg0 and/or HgCl2. Experimental results indicated that the saturated adsorptive capacity of Hg0 and/or HgCl2 increased with both the concentration and adsorption temperature. This suggests that the adsorption of Hg0 and/or HgCl2 on the composite sulfurized PACs follows a process of chemisorptions, with adsorption being favored at elevated temperatures. Additionally, when the adsorption occurred at 200–300°C, the saturated adsorptive capacity of Hg0 on the composite sulfurized PACs was higher than that of HgCl2. Using coal-fired power plants as an example, the saturated adsorptive capacity of single Hg0 or HgCl2 was always higher than that of dual Hg0 and HgCl2 when the molar ratio of influent HgCl2 to Hg0 was 5:5. In addition, the breakthrough time became shorter and the saturated adsorptive capacity decreased significantly, suggesting that competitive adsorption of Hg0 and HgCl2 occurred on the activated sites. Using municipal solid waste incinerators (MSWIs) as an example, where the molar ratio of influent HgCl2 to Hg0 was 6:4, the saturated adsorptive capacity of HgCl2 and/or Hg0 at the coal-fired power plants was significantly lower than that of the MSWIs at the adsorption temperatures of 150, 200, and 300°C. This shows that HgCl2 is a more competitive adsorbate than Hg0, which could tentatively be replaced by HgCl2 on the adsorptive sites. The experimental results indicate that the molar ratio of influent HgCl2 to Hg0 affected the viscosity of these two mercury species, as well as the saturated adsorptive capacity of dual Hg0 and HgCl2. The saturated adsorptive capacities of Hg0/HgCl2 found in this study were approximately 2.9–61.4 and 8.4–251.1 times higher than those in previous studies.