To investigate the origin of noncircular pores in the selective layer of asymmetric poly(vinylidene fluoride) (PVDF), membranes cast by immersion precipitation, the mass transfer kinetics were controlled by casting membranes at different temperatures and casting solution concentrations. Interconnected surface pore morphologies were observed by scanning electron microscopy (SEM) for membranes cast at high concentration and/or temperature, supporting a mechanism of liquid-liquid demixing at the membrane surface by spinodal decomposition. The coarsening of this highly porous, interconnected morphology leads first to chain-like formations of discreet, noncircular pores and ultimately to scattered pores of circular shape and large size dispersity. Similar results for poly(sulfone) (PSF), membranes, which solidify via vitrification rather than crystallization, suggest a degree of universality in the mechanism of morphological development of asymmetric membrane surfaces. This new understanding of pore formation kinetics opens the possibility of tailoring membrane surfaces to achieve a high degree of porosity and pore size uniformity.