In order to clarify the relationship between the porosity of membrane and the kinetics parameter of membrane formation (D.) in polysulfone (PSF) ultrafiltration membrane, phase-inversion process was used in PSF membrane preparation from PSF casting solution with poly(ethylene glycol) (PEG) or poly(vinyl pyrrolidone) (PVP) introduction. A new method used to calculate D,, was founded and an equation of D-a = d(2)/t was established on the basis of the relationship between wet membrane thickness and coagulation time. It was found that there was substantial relationship between D. and porosity in PSF ultrafiltration membrane, and that the linear relationship between D. and porosity in PSF ultrafiltration membrane could be described as P(%) = a + bD(a). (c) 2006 Elsevier B.V. All rights reserved.