The present work was concerned with skin layer formation kinetics of Polyethersulfone with Cardo (PES-C) asymmetric membranes by the phase inversion method. Gelation process was observed using an online optical microscope-CCD camera experimental system, the result indicated that a single linear correlation between square value of precipitation front move (X-2) and time (t) did not exist for the whole gelation process, which was different from the traditional conclusion. The effect of Ethylene glycol monobutyl ether (EGME) on skin layer gelation process was investigated, and the skin layer formation kinetics equation was determined, which could be described with gelation rate constant equation. The gelation rate constant decreased with the increase of concentration of EGME. EGME concentrations affected the viscous flow activation energy, and the increase of viscous flow activation energy led to the decrease of gelation rate. The change trend of the gelation rate was exactly the same as that of the flux. The results helped to understand the asymmetric membrane skin layer formation process better, which was important for controlling the membrane separation properties and designing the membranes for a specific application. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2392-2396, 2009