Nitrogen-based plasma systems such as N-2, NH3, Ar/NH3, and O-2/NH3 were used to modify microporous polyethersulfone membranes. Treatments were designed to alter the surface chemistry of the membranes to create permanently hydrophilic surfaces. Contact angle measurements taken initially, as well as 1 year post-treatment confirmed that treatments using O-2/NH3 plasmas (with a 5:3 gas flow ratio) were successful in achieving our designed goals. Analyses by FT-IR and XPS established the incorporation of NHx and OH species in die PES membranes. Moreover, the plasma penetrates the thickness of the membrane, thereby modifying the entire membrane cross-section. Optical emission spectroscopy studies of excited state species present in the modifying gases revealed the presence of OH*, which was not present in a 100% ammonia plasma, suggesting OH* must play a critical role in the membrane modification process. Investigations using bubble point analysis, differential scanning calorimetry, and scanning electron microscopy demonstrate there is no damage occurring under these specific treatment conditions. The usefulness of this treatment is revealed by increased water flux. reduced protein fouling. and greater flux recovery after gentle cleaning when compared to an untreated membrane. (C) 2004 Elsevier B.V. All rights reserved.