We have demonstrated, for the first time, that ultrathin skin-layer hollow-fiber membranes with a skin layer of 474 Angstrom can be prepared using mainly a one-polymer and one-solvent system. This is one of the thinnest skin-layer asymmetric hollow-fiber membranes that have ever been reported in the literature for air and gas separation. This work implies that, in order to yield a high-permeance polyethersulfone (PES) membrane with a skin layer of approximately 500 Angstrom, the addition of non-solvents into spinning dopes may not be the pre-condition to form ultrathin skin-layer hollow-fiber membranes for gas separation. The keys to fabricate ultrathin skin-layer hollow-fiber membranes are (1) to control the chemistry of the internal coagulant and the bore-fluid flow rate and (2) to have a dope exhibiting significant chain entanglement. The newly developed polyethersulfone (PES) hollow fibers have an O-2/N-2 selectivity of 5.80 with a permeance of 9.3 x 10(-6) cc(STP)/cm(2) s cmHg for O-2 at room temperature. The skin layer thickness was calculated to be 474 Angstrom. These hollow fibers were wet-spun from a 35/75 (weight ratio) PES/N-methyl-pyrrolidone (NMP) dope using water as the external coagulant and 80/20 NMP/H2O as the bore fluid. The hollow fiber must be coated with a silicone elastomer. This work also suggests that in order to yield a high-permeance PES membrane with a skin layer of approximately 500 Angstrom, there might not exist a critical solvent molar volume when preparing the dope solvent mixture, as previously suggested by the Permea research group. SEM observation of skin nodules suggests that the skin layer thickness is less than 700 Angstrom.