The impact of state of hydration and ion exchange on the exchange site local symmetry of Aquivion and Nafion perfluorinated sulfonated ionomers (PFSI) are probed by transmission IR spectroscopy.Hydrated PFSI-H membranes exhibit a pair of bands corresponding to a dissociated sulfonate exchange site with a local 3-fold axis of symmetry (C(3)v). C(3)v bands are supplanted by C-1 bands (no local symmetry) corresponding to the sulfonic acid form of the exchange site. At intermediate states of hydration C(3)v and C(1)bands co-exist Hydrated PFSI-Li exhibits C(3)v bands. In contrast to PFSI-H, the PFSI-Li C(3)v bands persist throughout dehydration, with a final aggregate structure where each Li+ provides 1/3 of a charge per sulfonate oxygen with an overall C(3)v motif. The C(3)v band FWHM values progressively increase (hydrated PFSI-H < hydrated PFSI-Li < dehydrated PFSI-Li) in the case of Nafion and Aquivion.FWHM trends are explained in terms of electric field induced exchange site polarization. The smaller Stokes Einstein radius of aquated Li+ enables closer proximity to the exchange site and thus greater induced band broadening. The relevant protonic Stokes Einstein radius must be derived from a tracer diffusion coefficient (D*): An effective protonic diffusion coefficient (Da) obtained from conductivity measurements (9.31 x 10(-5) cm(2)/s) yields a nonphysical Stokes Einstein radii of 0.26 A. A theoretically calculated D*, consistent with a group(-1) diffusion coefficient trend analyses, yields an aquated proton Haven ratio (D*/Da) of 0.086, a value typical of solid state fast ion conductors. The D* derived protonic Stokes Einstein radius of-3.1 A (relative to Li+ 2.01 A) is consistent with the ion-to-exchange site proximity argument for the observed FWHM trends. (C) 2017 Elsevier B.V. All rights reserved.