We have studied the nanoscale structure of a novel benzimidazolium-based ionene, methylated poly­(hexamethyl-p-terphenyl benzimidazolium) (HMT-PMBI), using a combination of X-ray scattering experiments and molecular dynamics simulations to explore samples with different levels of hydration and containing different anions. HMT-PMBI shows a remarkably simple morphology. At low levels of hydration, the bulky polymer backbones are unable to pack efficiently, leaving approximately 10% of the sample volume free of polymers, anions, or water. As the hydration levels are increased, water fills the free volume, forming a percolating network. This network forms at hydration levels as low as λ = 4 in samples containing less hydrophilic anions, such as bromide or iodide. At higher degrees of hydration, water simply forces the polymer backbones apart.