This review paper discusses the preparation of silicalite-l zeolite membranes, the experimental procedures used for gas separation measurements and the results of single gas and gas mixture experiments. 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. Transport through the silicalite membrane appeared to be controlled by molecular size and adsorption properties. Permeances of all components studied were activated, 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/SF6 and 1100 for H-2/i-C4H10 at 298 K. Separation selectivities at elevated temperatures were significantly above Knudsen diffusion selectivity 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 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.