Two types of commercial cellulose triacetate hollow-fiber membranes with different water fluxes, apposite for a reverse osmosis separation process, were investigated by means of positron annihilation lifetime spectroscopy with a Na-22-radioisotope-based energy-tunable positron beam, in order to elucidate a depth profile of the free-volume hole size in a subnanoscopic viewpoint. The lifetimes of ortho-positronium (o-Ps), the triplet positron-electron pair, for the two membranes were measured as a function of positron incident energy E. The E dependence of the hole size, quantified from the o-Ps lifetime, indicated that the separation-active dense layers with the smallest hole sizes are located at the near-surface region for both the membranes. The ratio of the dense-layer thicknesses for the two membranes was estimated to be more than 50% of the ratio of the respective effective thicknesses calculated from the water flux based on Darcy's law. The result signified that the dense layer with the smallest hole size at the near-surface region considerably contributes to the difference in the water flux. (C) 2014 Elsevier B.V. All rights reserved.