In this paper, the thickness-gradient membranes defined as flat sheet membranes with an increasing membrane thickness were prepared using polyetherimide (PEI)/N-methyl-2-pyrrolidone (NMP)/gamma-butyrolactone (GBL) casting solutions with the same polymer concentration (15 wt% PEI) and two different ratio of NMP/GBL by wet phase-inversion process. With an increase in membrane thickness, the membrane morphology evolution from sponge-like to finger-like structure was investigated clearly, and it was described with the critical structure-transition thickness (L-c) and general sponge-like structure thickness (L-gs). The morphology evolution of thickness-gradient membranes was illustrated with three different thickness regions. (C) 2008 Elsevier B.V. All rights reserved.