In this study, a new method of elongation was developed to measure the gelation time, a macroscopic time scale describing the gelation process during membrane formation. The gelation times of poly(ether suffones) (PES)/N-methyl-2-pyrrolidone (NMP)/nonsolvent membrane-forming systems were measured quantitatively for the first time. The effect of polymer concentration, approaching ratio and approaching coagulation ratio on the gelation time were systematically investigated. The results showed that the gelation time decreased with increasing approaching ratio, while increased with increasing polymer concentration in the casting solution and approaching coagulation ratio. For the membrane-forming systems studied herein, regardless of an instantaneous demixing system or a delayed demixing system, the gelation time was much longer than the delay time, and the gelation time changed in a similar pattern as the delay time. More efforts should be made to understand the role of gelation time in the process of membrane formation. (c) 2005 Elsevier B.V. All rights reserved.