We investigated the structure and the swelling behavior of two synthesized hydrocarbon polymer electrolyte membranes, made of multiblock copolymer poly(sulphonate phenylene)-b-poly(arylene ether ketone) with different block ratios, by using small-angle neutron scattering technique. A scattering maximum (ionomer peak) at high-q range (0.1 < q < 0.3 angstrom(-1)) is shown commonly in both dry and wet states, with q being the magnitude of the scattering vector, while it shifts towards low-q region in the wet state due to the swelling of the ionomer domains with water. The swelling effect also results to a second scattering maximum in the middle-q range (0.01 < q < 0.03 angstrom(-1)) because of the water-induced microphase separation. This swelling behavior was confirmed in various water mixtures of normal water and deuterated water with different volume ratios (contrast variation method). The morphology of the wet membranes was analyzed in terms of Hard-Sphere model with Percus-Yervick interference interactions. Our analysis indicated that (i) the hydrated microdomains in the membranes are interconnected, which is the key point to promote the proton conductivity; (ii) the water-induced microphase separation structure and the amphiphilicity of the matrix for embedding the ionomer domains are closely related to the chemical structure of the polymer. (C) 2016 Elsevier Ltd. All rights reserved.