Surface modifying macromolecules (SMM) were synthesized and blended into the casting solution of poly(ether sulfone). The solution was cast to films with thickness of 0.12 and 0.24 mm. The cast films were placed in an oven with forced air circulation for periods of 3, 5, 7, and 2000 min to remove the solvent, before being immersed into water at 4 degreesC for gelation. The membranes so prepared were further dried and subjected to contact angle measurement and XPS (X-ray photoelectron spectroscopy) analysis. It was found that the contact angle increased as the solvent evaporation period increased. The increase in contact angle was faster when the membrane was thinner. According to the XPS analysis, after an initial time lag the surface fluorine content increased as the evaporation time increased and finally leveled off. The increase in surface fluorine content was also faster when the membrane was thinner. A kinetic model was established for the SMM surface migration.