Block copolymer ionomers comprising an alternating sequence of hydrophobic and sulfonated hydrophilic blocks were synthesized successfully by a two-stage process: first the hydrophobic block was prepared with the desired length, then the monomers for the hydrophilic block were added at the desired stoichiometry to adjust the length. This approach provides control over the chemical structures of the resulting polymers and avoids the problems of post sulfonation. Phase separation of the block copolymers leads to physical crosslinking, leading to insolubility at elevated temperatures in water. Scanning electron micrographs of a dry sample indicates phase separation as expected. Preliminary fuel cell tests showed good stability as well as the acceptable performance of the membrane during the operation compared to Nafion((R)) membranes. (C) 2004 Elsevier B.V. All rights reserved.