Polymer electrolyte membranes are prepared from poly(arylene ether ketone) composed of hydrophobic and hydrophilic multi-blocks. 4,4'-bis(4-hydroxypheny1)-valeric acid and 4,4'-difluorobenzophenone are used for the synthesis of hydrophilic oligomers, while bisphenol A and 4,4'-difluorobenzophenone are used for hydrophobic oligomers. The proton conducting property is provided by sulfonation of the pendant carboxylic groups in hydrophilic blocks. The chemical structure of the oligomers and polymer synthesized is identified using H-1- and F-19- nuclear magnetic resonance (NMR) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopies as well as gel permeation chromatography (GPC). The development of distinguished hydrophobic-hydrophilic phase separation is confirmed by small-angle X-ray scattering (SAXS) spectroscopy and atomic force microscope (AFM). The thermal, mechanical, and chemical stabilities are endowed by the hydrophobic segment, whereas the proton conducting and water uptake properties are provided by the hydrophilic segment. The proton conductivity of the prepared membrane is comparable to that of Nafion 115 when the hydrophobic and hydrophilic block molecular weights are 3000 g mol(-1) and 18,000 g mol(-1), respectively. The block length effect on the membrane properties can be explained by the ion cluster dimension and structure examined by SAXS and AFM. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.