Although post-treatment modifications are widely employed to increase hydrophilicity and render membranes more biocompatible, they can also be used to modify the membrane pore size. In this study, hollow fiber Fresenius membranes made from polysulfone and poly(vinyl pyrrolidone) (PVP) were treated with a hypochlorite (bleach) solution for different periods of time. X-ray photoelectron spectroscopy (XPS) demonstrated a significant loss of PVP, with the bulk of the PVP removal occurring within the first hour of bleach exposure. The bleach-treated membranes also had a greater negative charge and a larger effective pore size. In contrast to the XPS data, the membrane charge and pore size continued to increase throughout the 14 h bleach exposure. Hindered diffusion and sieving data were analyzed using available hydrodynamic models that account for the asymmetric structure of the hollow fiber membranes. The good agreement between the hydrodynamic model and data indicates that the bleach-treated membranes were free of any large defects. (C) 2004 Elsevier B.V. All rights reserved.