Cellulose acetate (CA) membranes are prone to chemical attack mainly by operation at improper pH or by chlorine and other oxidizing agents. The chemical attack could lead either to hydrolysis of the pendant acetyl group or to oxidation of polymer backbone leading to chain scission. Hydrolysis can be detected by determining the degree of polymer acetylation, whereas oxidation can be detected by changes in polymer molecular weight as measured directly, for example, by the intrinsic viscosity measurement, and/or indirectly by determining polymer tensile strength. In the present study degree of acetylation, intrinsic viscosity and tensile strength measurements, were used for identifying the causes of the poor performance of two cellulose triacetate hollow fine-fiber membranes obtained from a commercial SWRO plant after being in operation for 5 years and 2 years, respectively To establish causes of degradation in the commercial samples, analyses were also performed on fresh virgin fiber, on a second sample exposed to high concentration of chlorine, and a third sample exposed to high pH. By comparison to the virgin sample, it was established that fiber exposure to a high concentration of chlorine and at high pH induced a decrease in polymer chain molecular weight and in degree of acetylation, respectively. Both the commercial SWRO membranes were found to have undergone hydrolysis as well as oxidation. However, polymer chain oxidation was established as the major cause for the degradation in performance of the commercial sample which was in operation for nearly 5 years, while hydrolysis of the pendant acetyl group was established as the major cause for the degradation in performance of the second commercial membrane sample which was in operation for 2 years. The paper describes the detail of the measurements made and results obtained.