In the present work, it is shown that addition of surfactants in the casting solution can induce or suppress macrovoids in polymeric membranes, depending on the miscibility between the added surfactant and the coagulant. Dual-bath experiments, in which casting solution was immersed in one coagulant for a very short time (<2 s) and then removed to a second bath containing different coagulants, were performed to investigate the formation process of macrovoids induced by surfactants. The results suggest that the initiation of macrovoids is completed in the first bath and the growth of macrovoids begins in the first bath but continues in the second bath. It was observed that the size of macrovoids is insensitive to the coagulant used in the second bath, which can be accounted for by proposing that the growth of macrovoids is mainly due to imbibing solvent from the surrounding polymer solution. To verify the above proposal, results from optical study and analysis of the growth speed of macrovoids are also presented.