Miscible acid-base blend membranes were developed by incorporating poly(ether ether ketone-alt-benzimidazole) (PEEK-alt-BI) into highly sulfonated poly(ether ether ketone) (SPEEK). The blend membranes with PEEK-alt-BI content up to 20 wt% via optical observation were transparent, indicating an excellent miscibility between the two components. This was further verified by a well-defined morphology and nanoscale domain size of the membranes according to TEM and SEM images. The compatibility between SPEEK and PEEK-alt-BI could be ascribed to the acid-base interactions between the sulfonic acid groups in SPEEK and the benzimidazole groups in PEEK-alt-BI as well as the similarity in the molecular structures of SPEEK and PEEK-alt-BI. It was found that, the addition of PEEK-alt-BI dramatically improved thermal stability and oxidative stabilities, dimensional stability, and mechanical properties of SPEEK membrane. The proton conductivity of SPEEK/PEEK-alt-BI-3 (85/15) membrane is up to 0.087 S/cm at 80 degrees C, which is higher than that of Nafion117 measured in the same condition. Meanwhile, its methanol permeability is nearly seven times lower than that of Nafion117 at 25 degrees C. (c) 2012 Elsevier B.V. All rights reserved.