Binary blends and their blend membranes of cellulose acetate (CA) and poly(vinyl butyral) (PVB) are prepared by solution blending. The compatibility of the blends is studied by viscometry and Fourier transform IR. It is found that the incompatibility of the blends is markedly manifested when the weight fraction of PVB in the CA/PVB blends (W-PVB) is located at higher regions. On the other hand, compatibility is obtained for the CA/PVB blends with lower W-PVB, values, especially at about 0.2. This compatibility is believed to play a key role in the good pervaporation behavior of CA/PVB blend membranes.