Electron spin resonance technique was used to study the structure and transport of the asymmetric polyethersulfone (PES) membranes. 2,2,6,6-tetramethylpiperridinyloxy (TEMPO) free radical was used as a spin probe that was brought into the membrane either by ultrafiltration experiments with feed solutions involving TEMPO, or by blending TEMPO into casting solutions. Ultrafiltration experiments were further carried out with polyethylene oxide feed solutions with and without TEMPO. It was found that two types of radicals exist in studied PES membranes, one located in the membrane pore and the other, which diffused into the polymer matrix. Unlike cellulose acetate (CA) membranes, which are more hydrophilic and swollen by water, the second type of radicals in PES membranes could not be leached out even after 24 It immersion in water. It seems that both pores and the water channels through the swollen polymer matrix contribute to the transport of water in CA membranes. On the other hand, the transport of water in PES membranes is primarily through pores and the contribution of the little swollen polymer matrix is insignificant. (C) 2002 Elsevier Science B.V. All rights reserved.