Formation kinetics of the poly(phthalazinone ether sulfone ketone) (PPESK) asymmetric membrane via wet phase-inversion process has been studied experimentally using an online optical microscope-CCD camera experimental system. This system has the ability to capture 12 frames of images per second with magnification up to 2000 times. The variation of the precipitation front movement, X, with time, t, has been measured from image analysis. By using Tikhonow regularization to extract the first derivative of X^2 verses t, i.e., dX^2/ dt, three different gelation steps have been identified. Each step corresponds to a formation process of the membrane morphology, namely the formation of the top layer, formation of the transition layer and formation of the support layer. For each gelation step, a straight linear correlation between the value of X^2 and t, can be obtained. However different gelation steps give different slope values. This is different from a commonly accepted conclusion which assumes a single linear correlation between X2 and t for the whole gelation process.