A structural study using both small-angle X-ray and neutron scattering (respectively SAXS and SANS) of water-swollen perfluorosulfonated ionomer (PFSI) membranes is presented. The comparison between the SAXS and SANS spectra combined with the analysis of both the scattering invariant and the Pored behavior of the scattering curves confirms the phase separation between the water peels and the perfluorinated matrix. The experimental data obtained in absolute units were analyzed in terms of the different models proposed for ionomers, and the local order model appears as the best model for describing the structure of swollen PFSI membranes. The size of the water pools and the number of ionic groups per ionic aggregate deduced from the analysis of the asymptotic behavior of the scattering curve at large angles compare well with the values deduced from the local order model. The SAXS spectra of swollen PFSI membranes are presented for the first time over an extended angular range covering 3 decades of angles using an ultra-small-angle scattering camera. The study of the small-angle upturn in intensity at very low angles revealed the existence of large-scale fluctuations.