Deuteron NMR spectroscopy was applied to study ammonium ion mobility below 50 K in fully deuterated ammonium hexachloroplumbate. Tunnelling frequencies from the spectra and correlation times from spin-lattice relaxation were measured. The spectra down to about 41 K show also a motionally narrowed component. The orders disorder phase transition takes place at 38.4 K. Below the phase transition two components are found and attributed to ions in ordered domains and transition regions between them, characterised below 25 K by tunnelling frequencies equal 0.35 and 1 MHz, respectively. A gradual change of the potential experienced by ammonium ions was observed. (C) 2002 Elsevier Science B.V. All rights reserved.