Effect of counter-anion on the properties and morphology of aliphatic poly(oxytetramethylene) ionenes was investigated, and the relationship between the properties and morphology was elucidated. The ionenes possess elastic properties at low elongations whereas they predominantly exhibit a plastic deformation at high elongations. The morphology of the ionenes was analysed in the nanometer scale by fitting a cascade model for randomly branched f-functional polycondensates incorporated in Debye-Bueche random-two-phase model and the interference term by the interdomain interaction on their small-angle X-ray scattering profiles. The size of ionic domains and distance between the domains were smaller and shorter in the ionene with bromide counter-anion than those in the ionene with chloride counter-anion, respectively. The fort-nation of crystallite of poly(oxytetramethylene) (POTM) segments was detected in the ionene with chloride counter-anion at room temperature. These characteristics of aliphatic POTM ionenes gave the difference in their mechanical and thermal properties. (C) 2004 Elsevier Ltd. All rights reserved.