Triamine-grafted pore-expanded mesoporous silica (TRI-PE-MCM-41) exhibited high CO2 and H2S adsorption capacity as well as high selectivity toward acid gases versus CH4. Unlike physical adsorbents such as zeolites and activated carbons, the presence of moisture in the feed enhanced the CO2 removal capability of TRI-PE-MCM-41 without altering its H2S adsorption capacity. Thus, depending on the feed composition, CO2 and H2S may be removed over TRI-PE-MCM-41 simultaneously or sequentially. These findings are suitable for acid gas separation from CH4-containing mixtures such,is natural gas and biogas.