Sorption of water in cellulose acetate membranes of two acetyl contents has been studied by infrared spectroscopy. A striking resemblance between the spectra of water in these membranes and those of water dissolved in ethyl acetate has been noted. It is concluded from the infrared spectra that, even near saturation, water sorbed in the membranes has a low degree of association and that bonds between water and cellulose acetate are considerably weaker than those in liquid water. Similar conclusions can be derived from the form of the sorption isotherms of water by cellulose acetate. The weak bonding of water to the membrane is consistent with its high mobility, while its low degree of association explains the low solubility of ions and hence the low permeability of the membrane to salts.