The potential of hybrid membranes as a CO2 capture technology for integrated gasification combined cycle applications was evaluated. Commercial gamma-alumina supports were modified with a variety of trichlorosilanes intended to enhance the surface adsorption of CO2. The resulting hybrids were characterized using X-ray photoelectric spectroscopy and Fourier transform infrared spectroscopy and tested for performance in the separation of He and CO2. The silanization temperature was determined to be important because membranes fabricated at 273 K had substantially different performance properties than those fabricated at room temperature. Specifically, the permeances of membranes modified with alkyltrichlorosilanes at reduced temperatures were 1-2 orders of magnitude higher than those of membranes fabricated at room temperature, and the selectivities of these low-temperature silanized membranes were relatively similar to those expected from Knudsen diffusion. Supports modified with silanes containing one of a variety of functionalities were tested for CO2/He selectivity. Membranes modified with 2-acetoxyethyl, 2-carbomethoxyethyl, and 3-aminopropyl groups exhibited CO2 selectivity, with the highest values approaching 7 for 2-carbomethoxyethyl- silated membranes at 50 degrees C. Temperature dependences resulted in selectivity maxima for the 2-acetoxyethyl and 2-carbomethoxyethyl membranes. Mixed-gas selectivities were slightly higher than pure-gas selectivities because of a decrease in He permeance with a relatively minor reduction in CO2 permeance. Transport in the selective membranes is believed to occur by a combination of activated and solution diffusion for He and a combination of activated and surface diffusion for CO2.