Abstract Several properties of short side chain perfluorosulfonate ionomer membrane films, made by the Dow Chemical Company, have been measured as a function of equivalent weight of the polymer. Water and electrolyte sorption and the self-diffusion coefficients of Na +, Cs +, Cl −, and I − have been measured. along with activation energies of diffusion. Also, the near infrared combination band for OH has been used to develop insight into the nature of sorbed water in the ion clusters of these micro-phase separated materials. Results show that these polymers have properties which are similar to Nafion ® perfluorinated ionomer membranes, with their diffusional properties being intermediate between those of the sulfonate and carboxylate polymers. Equivalent weight was found to influence the spreads in diffusion coefficients among ions, which increase with increasing equivalent weight. The sorbed water for these polymers show significantly reduced hydrogen bonding strength compared to bulk water, and this property is counter-ion dependent.