A series of block SPEEK copolymers with high ion-exchange capacities (IEC) consisting of hydrophobic and hydrophilic blocks were synthesized successfully. Membranes were cast from their DMF solutions, and characterized by determining the ion-exchange capacity, water uptake, water retention, proton conductivity and thermal properties. Membranes exhibited conductivities from 0.027 to 0.068 S cm(-1) at 30 degrees C and from 0.049 to 0.11 S cm(-1) at 80 degrees C; water uptake from 34% to 54% and IEC from 1.25 to 1.48 mequiv. g(-1). Block-3 membrane with IEC 1.38 mequiv.g(-1) showed relatively higher proton conductivity compared to that of a random SPEEK membrane with IEC 1.37 mequiv.g(-1). The SAXS profiles of block SPEEK copolymers exhibited a well-defined ionomer peak at low angles due to the phase separation between ionic domains and hydrophobic domains. It is believed that their microstructure features attribute to the enhanced proton conductivity of block SPEEK membranes. (c) 2006 Elsevier B.V. All rights reserved.