The kinetics of membrane formation by phase inversion was studied emphasizing the rate of solvent diffusion from a polymer solution during the phase separation. Diffusional behavior of the solvent can be considered Fickian. Membrane morphologies were shown to be strongly dependent on the rate of solvent diffusion, indicating that mass-transfer rates of solvent and nonsolvent during phase separation are crucial for determining the final membrane structure for the following system : polysulfone (polymer), dimethyl acetamide (solvent), and ethanol (gelation medium). Specific reference to the mechanism of macrovoid formation was explored. Macrovoid formation was found to be proportional to the square root of time, suggesting that it is governed by a diffusion process. In addition, latex particles of coagulated polymer formed by the nucleation and growth of a concentrated polymer phase was observed inside the macrovoids. Such a result implies that the macrovoids grow by a diffusive flow which results from the growth of the polymer lean phase during binodal decomposition.