The physical structure of polypent.enamer suHonat.e ionomers has been investigat.ed by small-angle neutron scatt.ering (SANS) and small-angle X-ray scatt.ering (SAXS). For a 17% cesium salt, a SAXS peak is observed in the dry stat.e at a value of the scatt.ering vector Q = 0.18, but this ionomer peak is undet.ectable by SANS. The addition of several wat.er molecules per salt group affords adequat.e neutron scatt.ering contrast to identify a scatt.ering maximum corresponding to the X-ray peak. SANS maxima are also observed in swollen 5.8% and 12% polypent.enamer ionomers as well as in hydrogenated ionomers. Higher wat.er cont.ents appear to effect a physical change in the wat.er-swollen ionomers-the peak position shifts to much lower angles and the peak int.ensity greatly increases. The SANS results are similar to those obtained from a fluorocarbon sulfonat.e ionomer and show behavior consist.ent with a phase-separated morphology.