The sulfonated polyether ether ketone (SPEEK) membranes containing poly(acrylic acid-co-4-vinylimidazole) (poly(AA-co-4VIm)) for low humidity operations in polymer electrolyte membrane fuel cells are focused. The blend membranes with homogeneous morphologies between SPEEK and copolymers are successful based on the hydrogen bond between SO(3)H or COOH groups and imidazole group observed by FTIR and SEM. The blend membranes also show an improvement in thermal stability as compared with SPEEK. IEC and proton conductivity of the blend membranes are increased with an increase of the acrylic acid content in poly(AA-co-4VIm). The dimensional stability of the blend membranes is significantly improved as evaluated by the swelling ratio. The present work shows how polymer chain with proton donor and acceptor functional groups plays an important role in enhancing proton conductivity together with other membrane properties. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.