A systematic study was carried out of the effect of the initial casting solution concentration, film thickness, and duration of the evaporation step on the occurrence and prominence of macrovoid pores in cellulose acetate membranes cast from acetone solutions via the dry/wet phase-inversion process. The duration of the evaporation step is shown to have a strong influence on macrovoid formation mainly through its influence on the solvent concentration gradient and casting solution viscosity at the instant of immersion into the precipitation bath. The results of this study are interpreted using predictive models for the evaporative casting process and for macrovoid formation.