Cellulose acetate (CA) is widely used in membrane processes. In this study, CA (weight-average molecular weight = 52,000) was mixed with poly(vinyl pyrrolidone) (PVP; weight-average molecular weight = 15,000) as all additive in 1-methyl-2-pyrrolidone as a solvent. The phase-inversion method was used for the preparation of flat-sheet membranes. The effects of PVP concentration and coagulation bath temperature (CBT) on the morphology, pure water permeation flux, and thermal stability of the prepared membranes were studied and are discussed in this article. The solute rejection of the developed CA membranes was quantified with an insulin protein solution. The results showed that an increase in the CBT levels from 0 to 23 degrees C along with all increase in the PVP concentration in the cast film from 0 to 1.5 wt % resulted in all increase in the macrovoid formation in the membrane sublayer, In increase in the pure water flux (PWF), and a decrease in insulin rejection. Further increases in the PVP concentration from 1.5 to 3, 6, and 9 wt % resulted in gradual Suppression of the macrovoid formation, a decrease in PWF, and all increase in insulin rejection. Higher PVP concentrations and lower CBT levels also appeared to result in higher glass-transition temperatures. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2537-2544, 2009