The aim of this work was to investigate anisotropic microporous membrane formation by phase inversion; using immersion precipitation technique and a ternary polymer system consisting of bisphenol-A polycarbonate, N-methyl-2-pyrrolidone as solvent and water as non-solvent to the polymer. Membrane morphology was observed by scanning electron microscopy (SEM) and related to some variables involved in membrane synthesis with the help of cloud-point measurements and precipitation kinetics, determined by means of light transmission experiments. The results show that the studied system presents a very small miscibility gap, which favors instantaneous demixing and the mechanism of nucleation and growth of the polymer lean phase, as well as macrovoid formation, yielding membranes with big macrovoids and low-pore interconnectivity, when directly immersed into a non-solvent bath. (C) 2000