In this study, the effect of HCl on the Hg0 removal performance for coal combustion flue gas system using activated carbons prepared from fly ash and pitch and activated carbon derived from coconut shell (a commercial sample) was investigated. Mercury (Hg0) removal capacity of fly ash-pitch activated carbon was higher than that of coconut shell activated carbon in the absence of HCl, however fly ash-pitch activated carbon showed lower mercury adsorption performance compared to the coconut shell activated carbon in the presence of HCl. It is suggest that in the absence of HCl, surface H2SO4 was produced by the adsorption of SO2, O2 and H2O on the activated carbon which then reacted with Hg0 to produce HgSO4 and enhanced the mercury adsorption capacity of the activated carbons. The presence of HCl in the feed gas increased the Hg0 removal performance of the activated carbon sorbents many fold. However, the presence of SO2 has an adverse effect on the adsorption capacity of the activated carbons in the presence of HCl.