Here, a new possibility for gas separation is presented, using an asymmetric membrane, with an ultrathin and defect-free skin layer. The structure of the asymmetric membrane, prepared by a dry-/wet-phase inversion process, showed an ultrathin skin layer and spongelike structure characterized by the presence of macrovoids. The gas selectivity of the membrane increases with a decrease in the thickness of the surface skin layer, perhaps because the membrane with a thinner surface skin layer forms a more packed structure. The effect of the membrane＇s surface skin layer on gas permeability and selectivity is discussed.