Polycrystalline (CH3)4NOH·5 H2O (I) and (CH3)4NOD·5D2O (II) have been studied by1H NMR lineshapes, second moments and spin-lattice relaxation times and by2H NMR lineshapes as a function of temperature. From low temperatures the first motion to occur is reorientation of the internally rigid (CH3)4N+ ion about a uniqueC3′ axis (Eta = 8.37 kJ/mol forI,Ea = 9.00 kJ/mole forII), followed closely by pseudo isotropic reorientation of the whole ion (Ea = 18.10 kJ/mol). Motion of the cage molecules (water and hydroxide ion) occurs at higher temperatures with an apparentEa = 11.30 kJ/mol. There is some evidence of a phase transition inII but notI in the 220–230 K region.2H NMR lineshapes ofII below 220 K indicate static cage molecules. Some of the2H quadrupole coupling constants derived from these spectra correspond to O·O hydrogen-bond distances which are incompatible with the known room temperature structure ofI. Above the possible transition inII the anisotropic2H lineshapes indicate rapid motion of2H among all possible hydrogen-bond sites via transfer along the bonds and molecular reorientation. This motion persists in the high temperature phase but the lineshape becomes isotropic due to the cubic symmetry of this phase. It is possible that1H or2H tunnelling plays an important part in the motion of the cage molecules and the different phase behaviour ofI andII.