Key parameters coupling with the instantaneous nucleation concept (ie, the Big Bang analogy) was used to model immersion precipitation process. The merits of the acquired model were verified via comparing its predictions with experimental results of two well-prepared and characterized cellulose acetate (CA) and polyacrylonitrile (PAN) membranes. A morphology predictable map, Delta P eta(-1) versus phi(1), was constructed, where Delta P, eta and phi(1) are osmotic pressure difference between nonsolvent and dope Solution, dope viscosity and intruded nonsolvent Volume fraction into the dope, respectively. The phase separation map, Delta P eta(-1) (proportional with apparent system diffusivity with the unit of time(-1)) versus phi(1) showed three regimes which, at least qualitatively, depicted the correct morphological evolution trends of the studied systems. Phase separation in regime one of CA membrane with the longest delayed time or lowest Delta P eta(-1), led to bead-like morphology. CA membrane with the shortest elapsed time or highest Delta P eta(-1), separated to finger-like morphology in regime three. Finally, phase separation in the intermediate regime of CA membrane, ended up to sponge-like morphology. Phase separation time scales of the PAN membranes versus intruded nonsolvent into the dope solution were located in finger-like region of the CA membrane, which its downward transition lowered the fingers population. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1529-1538, 2009