In this work we studied the effects of gas composition, particularly of H2S and SO2, on the Hg° removal performance of various un-impregnated activated carbons (ACs) using simulated coal combustion flue gas. Gas-phase Hg° removal experiments were carried out in a laboratory-scale fixed-bed reactor in a temperature range of 60-100 °C. The simulated flue gases had a composition of Hg° (4.9 ppb), H2S (0 or 1 or 20 ppm), SO2 (0 or 487 ppm), CO2 (10%), H 2O (4.7%), O2 (5%) and N2 (balance gas). ACs prepared from pitch and petroleum fly ash and coal fly ash exhibited a similar Hg removal capacity as the coconut shell AC. The presence of 20 ppm H2S and 487 ppm of SO2 in the simulated flue gases increased the Hg° removal capacity of the ACs significantly. However the effect of 1 ppm H2S in the presence of SO2 depended on the type of AC. Hg° removal capacity of coconut shell AC increased even in the presence 1 ppm H2S but there was no effect of 1 ppm H2S on the Hg° removal performance of fly ash ACs.