Viscosity slope coefficients (b) for cellulose acetate (CA) dissolved in four solvents tetrahydrofuran (THF), dioxane (DIO), acetone (AC) and butanone (MEK), for CA in THF at different temperatures and for CA/polyvinylbutyral (PVB) in DIO have been studied. The permeation flux (J) of the CA films from four solvents, CA film from THF at different temperatures and CA/PVB blended films from DIO for separating MeOH/MTBE (methyl tert-butyl ether) mixture with 3 wt.% MeOH were measured. It was found that the b and J values for CA, CA/PVB blends and their films were different, and the J values were inverse proportional to the b values. This opposite relationship between J and b could be due to the solvent solvating power, which alters b, and also the degree of coil overlap and the bulk density of polymer films.