In order to clarify the mechanism through which poly(ethylene glycol) (PEG) can affect the kinetics of membrane formation, phase-inversion process was used in membrane preparation from polysulfone (PSF) casting solution with PEG introduction. The theological and thermodynamic variation in PSF solution by PEG introduction was investigated by using the method of viscosity measurement and triangle phase diagram. It was found that PEG addition could work in favor of enhancement thermodynamically and hindrance theologically in PSF solution demixing. With the increase of temperature, the viscosity of PSF casting solution decreased and the miscibility of PSF solution with non-solvent increased. On the basis of the relationship between wet membrane thickness and coagulation time, a new method used to calculate kinetics parameter (D-a) Of membrane formation was founded and an equation of D-a = d(2)/t was established. With the increase of PEG concentration, it was revealed that D-a increased firstly and then decreased with a maximum value of 0.001428 mm(2)/s at certain temperature (20 degrees C). With the increase of coagulation bath temperature, Da increased continuously when the composition of casting solution was constant. D-a was mainly influenced by the surface morphology of membrane and the thermodynamic, theological properties of casting solution. (c) 2005 Elsevier B.V. All rights reserved.