Through-plane morphologies of multiblock copolymers based on poly(arylene ether sulfone)s were investigated by transmission electron microscopy (TEM) and through-plane conductivity measurements using a 4-probe method. The measured results showed that proton conductivity increased with a longer block length and that the optimal block length ranged from 5 to 7 K. TEM cross-sectional images also supported these results, indicating that a block length of 10 K is too well organized to conduct through-plane ion carriers. (C) 2009 Elsevier B.V. All rights reserved.