The mercury (Hg-0) removal process for coal-derived fuel gas in the integrated gasification combined cycle (IGCC) process will be one of the important issues for the development of a clean and highly efficient coal power generation system. Recently, iron-based sorbents, such as iron oxide (Fe2O3), supported iron oxides on TiO2, and iron sulfides, were proposed as active mercury sorbents. The H2S is one of the main impurity compounds in coal-derived fuel gas; therefore, H2S injection is not necessary in this system. HCl is also another impurity in coal-derived fuel gas. In this study, the contribution of HCl to the mercury removal from coal-derived fuel gas by a commercial activated carbon (AC) was studied using a temperature-programmed decomposition desorption (TPDD) technique. The TPDD technique was applied to understand the decomposition characteristics of the mercury species on the sorbents. The Hg-0-removal experiments were carried out in a laboratory-scale fixed-bed reactor at 80-300 degrees C using simulated fuel gas and a commercial AC, and the TPDD experiments were carried out in a U-tube reactor in an inert carrier gas (He or N-2) after mercury removal. The following results were obtained from this study: (1) HCl contributed to the mercury removal from the coal-derived fuel gas by the AC. (2) The mercury species captured on the AC in the HCl- and H2S-presence system was more stable than that of the H2S-presence system. (3) The stability of the mercury surface species formed on the AC in the H2S-absence and HCl-presence system was similar to that of mercury chloride (HgClx) species.