Ion-exchange membranes consisting of sulfonated polyether sulfone with controllable porosities and structures were prepared via a two-step phase inversion procedure. The porosity of membranes has been deliberately controlled by adjusting drying conditions. It was experimentally evidenced that membranes with high porosities possessed excellent conductivity; they also had poor selectivity and mechanical stability, while non-porous membranes exhibited much better selectivity and mechanical strength at the cost of lower conductivity. Porous membranes with 2.11 mequiv cm(-3) of fixed charged density, 0.33 mS cm(-1) of conductivity, 0.9 of transport number and similar to 500 MPa of Young's modulus were obtained by carefully controlling the two-step phase inversion preparation process. The results from this work lead to better understanding of the relationship among the formation conditions in water/dimethylformamide (DMF)/sulfonated polyether sulfone (sPES) system, structures and properties of membranes, which may shed light on advanced membrane design for appropriate applications. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.