The objective of this work is to present a simple mathematical model to describe the main features of phase inversion by immersion precipitation. The model was developed for planar geometry and is used to simulate membrane synthesis. Two systems commonly used for membrane preparation were used for simulations: cellulose acetate/acetone/water and polyetherimide/N-methylpyrrolidone/water. The influence of nonsolvent addition to initial polymer casting solutions, solvent addition to the coagulation bath, and geometric variables, such as polymer film thickness, on the Final precipitation conditions were studied through simulation and compared to available experimental data. The results are in good agreement with published results.(1-3) It is shown that polymer film composition profiles at the moment of precipitation may give important information about the structure and substructure of formed membrane. It is also shown, for both polymeric systems investigated in this work, that the dynamics of the mass transfer process seems to be much more important than the influence of the concentration on the diffusion and thermodynamic partition coefficients, as fair agreement with available experimental data was obtained even when these coefficients were assumed to be constant.