The diffusivity, solubility, and permeability of water vapor and nitrogen were systematically investigated for Vacuum Ultraviolet (VUV)-irradiated poly(lactic acid) (PLA) membranes by a xenon excimer lamp at 172 nm. The photon was completely absorbed in the surface membrane, and the VUV irradiation did not depend on the color center and bulk structure of the PLA membrane. Additionally, surface hydrophilicity increased in the irradiation time range of 0-10 min and settled at a steady state in the irradiation time range of 30-100 min. We discovered the surface photo-oxidation, scission reaction, and crystallization of PIA membranes by VUV irradiation. The PLA chemical structure changed as chain cleavage and formation of C=C double bonds and hydroperoxide O-H at newly formed chain terminals through a Norrish II mechanism. The water vapor permeability of VUV-irradiated PLA membrane for 60 min was equal to that of unirradiated PLA membranes because the water vapor diffusivity reduction for surface crystallization counteracted the solubility enlargement for surface hydrophilicity. The temperature dependence on water vapor permeability of VUV-irradiated PLA membrane at low relative feed pressure showed that diffusivity-controlled behavior and that, at high relative feed pressure, diffusivity- and solubility-controlled behavior. (C) 2011 Elsevier B.V. All rights reserved.