We prepared a polymeric membrane with a fine porous structure from polystyrene (PS), poly(ethylene glycol) (PEG), and solvent solutions by exploiting the phase separation induced in the Course of dry casting. To determine the effect of the drying rate and phase separation on the developed porous structure, six different solvents, including toluene, chlorobenzene, tetrahydrofuran, methyl ethyl ketone, 1,4-dioxane, and chloroform, were used. The pore size and density drastically changed with the different solvents and drying conditions. The polymer concentration at the onset of the phase separation into PEG-rich and PS-rich phases also strongly affected the cellular structure. The solubility of PEG into PS and the solvent Solutions changed the concentration, which corresponded to the viscosity of the PS-rich solution at the onset of the phase separation. The higher solubility of PEG in the solutions delayed the onset of phase separation during drying and increased the viscosity. The higher viscosity and the higher drying rate prevented the phase-separated PEG domains from coalescing and made the resulting pore size smaller and the pore density larger. The finest porous structure, with a pore size of approximately 1 mu m and a pore density of 0.08 1/mu m(2), was prepared from PS/PEG and a 90 wt % chloroform solution. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2518-2526, 2009