The factors contributing to the selective permeation of water and sodium chloride through cellulose acetate membranes have been examined by the use of radioactive tracers. With decreasing acetyl content both the partition coefficient (solubility) and diffusion coefficient of water increased, the latter the more sharply. The effect was even more pronounced for salt, indicating that increasing selectivity with acetyl content stems mainly from increasingly preferential restrictions on salt mobility. Trends identical with those mentioned for decreasing acetyl content were found for increasing amounts of cellulose acetate solvents that had been extracted with water to yield more highly swollen membranes. A free-volume treatment for diffusion of small molecules below the glass transition temperature with the aid of subgroup motion in the polymer is used for both components. The water content of the membrane at (or near) saturation emerged as the predominant factor in the permeation behavior. In view of the similarity in the activation energies of water and salt diffusion the far steeper dependence of the salt diffusion coefficient on water content could not be accounted for by size differences between the diffusing species and has been attributed to confinement of salt ions to locations at which multiple water contacts are feasible.