Perfluorosulfonate ionomers (PFSIs) neutralized by tetraalkylammonium ions have been investigated using variable temperature H-1 and C-13 solid-state NMR (ssNMR) spectroscopy to probe the thermally induced properties of tetraalkylammonium ions at temperatures near the a-relaxations. Tetramethylammonium (TMA(+)), tetraethylammonium (TEA(+)), tetrapropylammonium (TPA(+)) and tetrabutylammonium (TBA(+)) ions have been incorporated in our study for the systematic control of ionic interactions within PFSIs according to the chain-length of ammonium ions. C-13 static ssNMR results show that bulkier TPA(+) and TBA(+) ions undergo molecular tumbling motions near or above the alpha-relaxation temperature of the corresponding PFSI ionomers, with jumping rates of kappa approximate to 1 kHz. Moreover, the results of H-1 spin-lattice (T-1) relaxation time measurements suggest that smaller TMA(+) ions in the PFSI exhibita thermally induced ordering effect as the sample temperature approaches the alpha-relaxation temperature of TMA(+)-PFSI. This ordering phenomenon is also supported by the results from small-angle X-ray scattering. (C) 2009 Elsevier Ltd. All rights reserved.