The formation process and morphology of BTDA-TDI/MDI co-polyimide (P84, CAS#: 58698-66-1) asymmetric flat sheet membranes have been studied. Experimental results indicated that the weight ratio of H2O and N-methyl-2-pyrrolidone (NMP, CAS#: 872-50-4) at cloud point curve (above critical point) was a constant (7.66/92.34 (w/w)) for the P84/NMP/H2O system. For two different casting solutions (21 wt.% P84 in pure NMP; 15 wt.% P84 in H2O/NMP: 6.0/94.0 (w/w)), the approaching ratio a strongly dominated the formation of finger-like structure rather than the viscosity of casting solution. The formation of finger-like structure in P84 co-polyimide asymmetric membranes was due to the hydrodynamically unstable viscous fingering developed when the casting solution was displaced by a polymer-lean phase. Three types of membrane morphologies, finger-like structure, transition structure and sponge-like structure can be expected with various approaching ratio alpha of casting solutions. The critical approaching ratio alpha* was initially defined to describe the sharp change of membrane morphology from finger-like to sponge-like structure. The casting temperature also influenced the membrane morphology. For some casting solutions (e.g. 15 wt.% P84 in H2O/NMP: 6.4/93.6 (w/w)), the membrane morphology changed from sponge-like to finger-like structure with an increase in casting temperature. Meanwhile, the critical approaching ratio alpha* also increased with an increase in casting temperature. (C) 2004 Elsevier B.V. All rights reserved.