We have investigated the macromolecular structure and theological behavior of both linear and hyperbranched polyethersulfone (PES) materials. It was found that the hyperbranched PES material has a higher molecular weight and a wider molecular weight distribution than its linear analogue. Rheological studies disclose that polymer solutions made from the HPES/polyvinylpyrrolidone (PVP)/N-methyl-2-pyrrolidone (NMP) ternary system have a longer relaxation time than their linear counterparts. The less relaxation characteristics of the HPES dope not only result in a more pronounced die swelling during hollow fiber spinning, but also produce hollow fiber membranes with smaller pore sizes, narrower pore size distribution, and a smaller molecular weight cut-off (MWCO). In addition, elongational viscosity characterizations indicate that HPES possesses a more strain hardening effect than LPES. As a result, films made from the former tends to break easier and quicker under high extensional strains than those made from the latter. (C) 2008 Elsevier Ltd. All rights reserved.