High temperature H2 separation membranes may play an important role in future cost-effective production of H2 from catalytic reforming of coal, natural gas, and biomass for use in the emerging fuel cell-based automobiles and clean power systems. alpha-Alumina-supported MFI zeolite membranes were modified by catalytic cracking of methyldiethoxysilane (MDES) molecules inside the zeolitic channels on-stream during separation of H2/CO2 gas mixture at 450°C. The MDES vapor was carried by the H2/CO2 feed stream and the effect of modification was monitored continuously through analysis of the permeate stream. A significant increase in H2 selectivity over CO2 was achieved with a moderate loss in H2 permeance on the modified membrane. The modified membrane showed good stability in separation of H2/CO2 gas mixture containing water vapor at 450°C and 1 atm. This is an abstract of a paper presented at the AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).