The relationships between the nano-scale structure and the monomer composition of proton exchange membranes (PEMs) are reported. The PEMs are synthesized by preirradiation-induced grafting ETFE with styrene and cross-linker, 1,3-diisopropenylbenzene (DIPB), where the styrene moieties are sulfonated subsequently. The degree of grafting is constant, while the DIPB level is varied systematically. The SAXS spectra of the dry membranes are isotropic and dominated by a single correlation peak, which results from the phase separation of the ion-rich phase from the polymer matrix. By analyzing the correlation peak we find that the number density and the typical size of the ion-rich domains decrease when the level of cross-linking is increased. The proton conductivity in the fully hydrated state is proportional to the volume fraction of the ion-rich phase. This suggests that the structure of the ion-rich phase found in the dry state has fundamental impact on the conductivity of the hydrated membrane. The relationship between the proton conductivity and the water volume fraction follows a power law in good agreement with percolation theory.