Polyethersulfone (PES)/cellulose acetate phthalate (CAP) blend ultrafiltration (UF) membranes were prepared with phase inversion induced by immersion precipitation method. CAP was employed to improve the hydrophilicity of PES membranes. Polyvinlylpyrrolidone (PVP) was selected as pore former. Dimethylacetamide (DMAc) and water+isopropyl alcohol (IPA) (80/20 vol%) were used as solvent and coagulant respectively. The contact angle measurements indicate that the hydrophilicities of PES/CAP membranes are enhanced by increasing the CAP concentration in the casting solution. However this increment does not improve the membrane performance in all cases. The AFM and SEM images reveal that addition of low concentration of CAP in the casting solution results in a membrane with high surface and sub-layer porosity and thin top layer. The pure water flux and milk water permeation for the prepared membranes are increased by introducing 20% CAP in the casting solution. However further addition of CAP sharply decreases the membrane performance. The protein rejection is slightly increased to 99% for 80/20 blend of PES/CAP. This falls to 50% at 60/40 of PES/CAP composition. Furthermore, it was found that the low concentration (2 wt.%) of PVP in 80/20 percent of PES/CAP in the casting solution results in a membrane with higher mechanical strength and better performance. CAP may play a role as an as antifouling agent. (c) 2007 Elsevier B.V. All rights reserved.