Following an inspiration from Vogrin et al. (J. Membr. Sci. 2002, 207, 139), we have investigated the thickness dependence of macrovoid evolution during the phase inversion of asymmetric flat membranes and observed, for the first time, a critical structure-transition thickness, L, which indicates a transition of the membrane morphology from a spongelike to a fingerlike structure with an increase in membrane thickness. Below L, the membranes show a fully spongelike cross-sectional structure, whereas above L, the membranes exhibit a mainly fingerlike macrovoid structure. Two binary solutions are employed in this study: 20 wt % polyethersulfone/N-methyl-2-pyrrolidone (NMP) and 20 wt % BTDA-MDI/TDI co-polyimide/NMP solutions. Their corresponding critical thicknesses, L-c, are 1.5 +/- 0.4 and 11 +/- 2 mum, respectively.