The transitions of several symmetric tetraalkylammonium halides were studied by differential scanning calorimetry (DSC), solid-state nuclear magnetic resonance (NMR) and X-ray diffraction. The reversibility of the transitions of Bu 4NI is investigated by cycling of the samples in the DSC. A plastic-crystalline phase is found and characterized. A glass transition is observed for Bu 4NCl at 230 K. Solid-state CP-MAS 13C NMR is performed on Hex 4NI in order to reveal the large-amplitude molecular motion. The DSC transitions are confirmed by changes in the chemical shifts, but additional, more continuous changes are seen. Condis-crystalline phases are probable for the latter two salts. Room-temperature X-ray diffraction on various iodides shows that the appropriate d-spacings increase linearly with size of the alkyl groups.