Large, high-quality, single crystals of pure-silica ferrierite are synthesized, and the structure is described. Selected individual crystals (approximately 600 mu m X 500 mu m x 20 mu m) are mounted in a membrane configuration so that only the 10-membered-ring channels (5.4 Angstrom x 5.4 Angstrom x 4.2 Angstrom) or the 8-membered-ring channels (4.6 Angstrom x 3.7 Angstrom X 3.0 Angstrom) are accessible for gas-molecule permeation. The first examples of transport exclusively through 8- or 10-membered-ring channel systems are reported and obtained through crystal orientation in the membrane. A series.of adsorption experiments are conducted to help select suitable probe molecules and evaluate the role of adsorption lit the permeation process for single-crystal membranes. Methane, n-butane, isobutane and nitrogen probe molecules are used to study intracrystalline sorption and transport effects for different crystal orientations, pressures and temperatures. Both pure-gas selectivities and mixed-gas separation factors are reported. A,mixed-gas separation factor of n-butane/isobutane = 116 for the 10-membered-ring orientation of the crystal at 383 K and a transmembrane pressure difference of 1.01 x 10(5) Pa are found using this technique. In addition, molecular sieving is observed for the 8-membered-ring orientation of the crystal since methane, but not butane, transport is observed for this crystal orientation.