Formation kinetics of the poly (phthalazinone ether sulfone ketone) (PPESK) asymmetric membrane via wet phase-inversion process has been studied experimentally. The membrane morphology has been observed using an online optical microscope-CCD camera experimental system. The precipitation front movement, X, has been measured. Three different linear correlations between the value of X-2 and the gelation time, t, have been identified. This observation is different from a commonly accepted conclusion which assumed a single linear correlation between X2 and t for the whole gelation process. Compared to the morphology evolution of the membrane, it is realized that these three correlations correspond to the three consecutive gelation steps: formation of the top layer, formation of the transition layer and formation of the support layer. The effect of two additives, PEG 1000 and Tween80, on the formation kinetics as well as the membrane flux has also been studied. The results present here may provide better understanding of the asymmetric membrane formation process. (c) 2005 Elsevier B.V. All rights reserved.