Permeabilities to three gases (carbon dioxide, methane, and nitrogen), water vapor, and five solvents (methanol, acetone, cyclohexane, xylene, and chloroform) are reported for a broad range of commercial polymeric membranes. Gas and water vapor transmission (WVT) data were determined by ASTM methods and solvent vapor transmission data were obtained in a modified WVT procedure. Permeability characteristics of thermoplastic and partially crystalline membranes were also assessed in pouch-type tests with salt solutions, actual wastes, acetone, xylene, and water-soluble and organic-soluble dyes. All membranes were permeable to some extent. Magnitude and direction of fluid transport vary with the membrane composition and its thickness, temperature, the permeant fluid, and the driving force which depends upon the concentration or the vapor pressure gradient across the membrane. Permeability depends upon the solubility and diffusion characteristics of the permeant in the particular membrane.