A mixture of equal parts of cellulose diacetate and cellulose triacetate was dissolved in dipropylene glycol and exposed to shear stresses of varying intensity on a three-roll calander. Asymmetric reverse osmosis membranes were prepared from these materials by the phase-inversion method. Reverse osmosis tests in a dead-end module provided membrane performance data. A structure analysis was performed by scanning electron microscopy and the microstructure of the membranes was investigated by differential scanning calorimetry and X-ray diffraction. It was observed that increasing shear times as well as shear rates reduced the salt rejection while the permeate flux was escalating. The size-exclusion chromatography analysis showed a strong decrease of Mw and Mz at a constant Mn indicating the degradation of the higher molar mass macromolecules. Since the physical structure of the membranes was not affected and a change in polymorphism could not be related to the variations in the reverse osmosis performance the reduced salt rejection should be caused by this degradation process, supposedly through affecting the material's diffusion properties by changing the fractional free volume.