Silicalite-alumina composite membranes were prepared by an in situ zeolite synthesis method using an alumina membrane tube with a 5-nm pore diameter, gamma-alumina layer as a substrate. Single gas permeances of H-2, Ar, n-C4H10, i-C4H10, and SF6 were measured and mixtures of H-2/i-C4H10 and H-2/SF6 were separated to characterize the silicalite membrane. These measurements were made from 300 to 737 K, and are compared to an alumina membrane without a silicalite layer. Permeances were lower in the silicalite membrane (a factor of 8 for Ar at 298 K). Permeances for the alumina membrane decreased as the temperature increased, and separation selectivities were lower than values expected for Knudsen diffusion. Transport through the alumina membrane was by Knudsen flow and surface diffusion. The silicalite membrane showed dramatically different behavior, and transport appeared to be controlled by molecular size and adsorption properties. Permeances of all components studied were activated in the silicalite membrane, and activation energies ranged from 8.5 to 16.2 kJ/mol. The ratio of single gas permeances was as high as 136 for H-2 to SF6 and 1100 for H-2 to i-C4H10 at 298 K. Separation selectivities at elevated temperatures were significantly above Knudsen diffusion selectivity for the silicalite membrane and were larger than ratios of pure gas permeances at the same temperature. The largest permeance ratio for the separation of mixtures was 12.8 for H-2/SF6 at 583 K. Separation selectivities for both membranes were higher when a pressure drop was maintained across the membrane than when an inert sweep gas was used because of counter diffusion of the sweep gas.