In this paper. asymmetric membranes are prepared with P84 (BTDA-TDI/MDI co-polyimide)/N-methyl-2-pyrrolidone (NMP)/-gamma-butyrolactone (GBL) and polyetherimide (PEI)/NMP/GBL. casting solutions by wet phase-inversion process And the transition of membrane morphology is investigated with different flat sheet and thickness-gradient membranes. For the thickness-gradient membranes, with the increase of membrane thickness, the membrane morphology changes from sponge-like to finger-like structure at the critical structure-transition thickness L-c. Meanwhile, the general sponge-like structure thickness L-gs underneath the membrane surface is almost a constant. And the morphology of thickness-gradient membranes IS Illustrated with three different thickness regions. For the flat sheet membranes prepared with PEI/NMP/GBL casting Solutions. with the increase of GBL. weight ratio w(GBL), the transition of membrane morphology from finger-like to sponge-like structure occurs it the critical weight ratio of GBL (w* = 0.588) The presence of GBL increases the viscosity of the casting solutions and suppresses the tendency of macrovoid formation. For the thickness-gradient membranes prepared with PEI/NMI/GBL casting solutions, the presence of GBL retards the transition of membrane morphology The increase of w(GBL), results in the increase of L-c and L-gs. When WGBL is greater than w*, the transition of membrane morphology is not investigated and a fully sponge-like structure is formed (C) 2009 Elsevier B V All rights reserved.