Sponge-like and finger-like structures are two distinct membrane structures commonly observed in membranes produced by the wet immersion process. An index phi calculated solely from solubility parameters was defined as an indicator of the membrane structure. The phi values of four polymers, poly(methyl metharylate), polysulfone, cellulose acetate, and poly(vinylidene fluoride), in various solvent-nonsolvent pairs were calculated and compared with the corresponding membrane structures. It was found that the finger-like structure often occurred at higher phi values. Although the phi value represents mostly the thermodynamics nature of a system, as an index for prior selection of solvent-nonsolvent pairs for a particular polymer, a general rule of thumb was developed. Taking 15% polymer concentration and 300 mu m casting thickness as a referential casting condition, selecting solvent-nonsolvent pairs with phi values higher than 0.25 is suggested, when a finger-like structure is desired. The polymer concentration in the casting solution and the casting thickness will also affect the membrane structure. If a higher polymer concentration needs to be used, selection of a polymersolvent-nonsolvent system with a phi value much higher than 0.25 is suggested, or keeping the casting thickness lower than 300 mu m to obtain a finger-like membrane structure.