Phase transition and ionic transport in tetrabutylammoniumiodide (C4H9)(4)NI (TBAI) was investigated using X-ray diffraction, DSC, ionic conductivity and high resolution proton NMR spectroscopy. The TBAI exhibited a solid state phase transition at 391 K of OF Delta S = 95.2 J/mol K from an ordered phase I to a rotationally disordered plastic phase II. Ionic conductivity in phase I increased gradually near the phase transition point. On the other hand, proton NMR diffusion coefficients showed decrease with raising temperature. The observed fast diffusion of the TBA cation has significant contribution to the observed ionic conductivity of this material. (c) 2005 Elsevier B.V. All rights reserved.