Membrane formation from poly(bisphenol-A sulfone)/poly(vinyl pyrrolidone)/dimethylacetamide/water systems by phase inversion process using immersion-precipitation technique was investigated. The initial precipitation rate was determined from light transmission experiments and the membranes morphologies were observed by scanning electron microscopy (SEM). These results were used to explain an observed oscillatory behavior in macrovoid occurrence, as well as to identify the region where spinodal demixing dominates the early stages of the phase inversion process. It is proposed as a qualitative model dividing the solution in three different layers during the polymer solution mass exchange with the coagulation bath. Each layer is associated with different precipitation kinetics leading to distinct morphologies. The model assumes that the macrovoids development is a function of the resistances created by precipitation kinetics of former layers.