Poly(dibenzylated benzimidazolium) bromides (Bz-PBI-Br) were converted successfully to OH- ion conducting poly(dibenzylated benzimidazolium) hydroxides (Bz-PBI-hydroxides) by the treatment of KOH. The Bz-PBI-hydroxides obtained in this study showed an excellent alkali tolerance compared to previously synthesized poly(dimethylated benzimidazolium) hydroxides (Me-PBI-hydroxides). According to H-1-NMR analysis, Me-PBI-hydroxides were decomposed during KOH treatment. In order to find out the reason, density functional theory (DFT) calculations of two benzimidazolium structures, e.g., di-methylated benzimidazolium (Me-BI+) and dibenzylated benzimidazolium (Bz-BI+), were performed. Bz-BI+ showed lower electron affinity and OH--binding energies at the C2 position of the benzimidazolium ring than Me-BI+ These DFT results strongly confirm that Bz-BI+ is less vulnerable to an OH attack than Me-BI+; this contributes to the enhanced stability and OH- ion conductivity of the Bz-PBI-hydroxides. (C) 2016 Elsevier B.V. All rights reserved.