To modify the adsorption characteristics of the mesoporous silica FSM-16, Al-incorporated FSM -16 (Al-FSM-16) and trimethylsilylated FSM-16 (TMS-FSM-16) were prepared. The pore structure and the adsorption characteristics of these modified mesoporous materials were examined by using probe molecules Of N-2 and CO2. BET surface area of Al-FSM-16 was reduced to 470 m(2)-g(-1), about one-half of the surface area of FSM-16. However, the separation factor for CO2 from CO2-N-2 gas mixtures (CO2/N-2=20.1/79.9) on Al-FSM-16 was higher than on FSM-16 in the total pressure range from 0.2 to 0.3 MPa. This implies that CO2 is adsorbed selectively to Na, which is present to compensate for the negative charge generated by incorporating Al in the framework of the mesoporous silicas. By trimethylsilylation of FSM-16, 82% of the surface area was covered with trimethylsilyl (TMS) groups, and the pore size decreased to 2.3 nm from the 2.8 mn of FSM-16. Much less CO2 was adsorbed on TMS-FSM -16 than that on FSM-16, possibly due to the smaller amount of free SiOH groups of TMS-FSM-16.