The present study employs small-angle X-ray scattering (SAXS) to investigate the water-swollen structures of two types of Nation membranes, commercial Nation H 7 membranes and the membranes synthesized from the Nafion precursor, subjected to gamma radiation. The membrane structure can be characterized by two-length scales, comprised the long-range order of lamellar crystalline domains in the matrix and the local order of ionic cluster domains. Both the long-range order lamella and local order cluster in the membranes are significantly affected by the radiation-induced cross-linking. We have extended a local order model to take into account the polydispersity effect, which can more satisfactorily reproduce the ionomer SAXS peak than other existing models. The structural parameters determined from the SAXS model analysis are self-consistent with those obtained from the model-independent Porod analysis. The modified membrane structure by the radiation cross-linking is very helpful for developing a high performance and low cost of direct methanol fuel cells. (c) 2005 Elsevier Ltd. All rights reserved.