A novel lower critical Solution temperature (LCST) membrane forming system containing polyethersulfone (PES)/poly(vinyl pyrrolidone) (PVP) K90/N, N-dimethylacetamide (DMA)/12-propanediol (1,2-PD) with a mass ratio of 15/5/65/15 had been developed. A LCST dope had the advantage of ease to control the dope close to its cloud point and meanwhile to maintain the solution at a stable state compared to an UCST one. Hollow fiber ultrafiltration (UF) membranes were prepared by using a wet spinning process. The membranes were post-treated by means of a hypochlorite solution of 4000 ppm over 48 h. The experimental results showed that the membrane flux was increased by live times and the pore size distribution became narrow after the hypochlorite treatment. SEM images revealed that PVP contents in the blend membrane could be partially removed with the hypochlorite treatment and that the thickness of the dense skin layer was 0.5 mum after hypochlorite treatment. It was concluded that a dope formulation being very close to its cloud point would result in a membrane of a thin skin with high flux. A PES hollow fiber UF membrane with a water flux of 1565 X 10(-5) L m(-2) h(-1) Pa-1 was obtained via the technique described in this work. (C) 2004 Elsevier B.V. All rights reserved.