The state of water in the active layer of a series of cellulose acetate asymmetric membranes, prepared by the phase inversion process, was investigated using attenuated total reflection infrared spectroscopy (ATR-FTIR). The relative amount of water present in the active layer depends oil the inherent surface or skin layer morphology; the more permeable ultrafiltration asymmetric membranes contain more water compared to the less permeable nanofiltration/reverse osmosis asymmetric membranes studied. Lightly clustered water species, weakly hydrogen-bonded to the polymer hydroxyl groups, predominate in the skin layer of the reverse osmosis membranes whereas the ultrafiltration membranes contain predominantly bulk-like water clusters. During the rehydration of tile dried membrane, either with liquid water or in a constant relative humidity environment, the size of the water clusters and the extent of the water-water interactions in these clusters increase as more water is sorbed by the active layer. This difference in the state of water (amount and type of water species) present in the skin layer evidences the heterogeneous morphology of the active layers for a series of cellulose acetate asymmetric membranes prepared by the phase inversion technique.