A simple equation has been developed to determine the relationship between the mass ratio of nonsolvent-additive to solvent (R(a/s)) in a membrane casting solution and its coagulation value (G(v)) in preparing asymmetric membranes by the wet or the dry/wet please inversion process. The equation is based on the precipitation values of the nonsolvent-additive (PVa) and the coagulant (PVc) in the binary polymer/solvent system. The validity of this equation was examined with observed experimental results for PESf/NMP/alcohol, PESf/NMP/H2O, PSf/DMAC/alcohol and PSf/DMAC/H2O polymer casting solutions using water or ethanol as the coagulant; and also with the experimental data reported in the literature for the PSf/NMP/PA system using water as the coagulant. The results show that the developed equation describes the observed experimental data very well. This equation may be used to indicate variation of phase separation behavior of the membrane casting solution with a coagulant by the introduction of a nonsolvent-additive in the solution. It can be applied readily for the formulation of composition of the membrane casting solution to make flat sheet or hollow fiber membranes. The equation may be useful in the study and control of membrane morphology.