Boron-substituted ZSM-5 and SAPO-34 membranes were silylated by the catalytic cracking of methyldiethoxysilane (MDES) to increase their selectivity for H-2 separation from light gases. The MDES reacted in the B-ZSM-5 pores and reduced their effective pore diameter, so that silylation significantly increased their H-2 Selectivity. The H-2/CO2 separation selectivity at 473 K increased from 1.4 to 37, whereas the H-2/CH4 separation selectivity increased from 1.6 to 33. However, silylation decreased the H-2 permeances more than 1 order of magnitude in the B-ZSM-5 membranes. The H-2 permeance and H-2/CO2 and H-2/CH4 separation selectivities increased with temperature. At 673 K, the H-2 permeance was 1.0 x 10(-7) mol(.)m(-2.)s(-1.)Pa(-1) and the H-2/CO2 separation selectivity was 47. Methyldiethoxysilane does not fit into SA-PO-34 pores, but silylation apparently decreased the pore size of the nonzeolite pores in the SAPO-34 membranes. After silylation, the H-2 permeances and H-2/CO2 and H-2/N-2 separation selectivities were almost unchanged in the SAPO-34 membranes because H-2, CO2, and N-2 permeate mainly through SAPO-34 pores. In contrast, H-2/CH4 separation selectivity increased from 35 to 59, and CO2/CH4 separation selectivity increased from 73 to 110, apparently because CH4 permeates mainly through non-SAPO-34 pores.