The temperature dependence of the phase memory time T-M in gamma-irradiated lithium hydrazinium sulphate, LiN2H5SO4, single crystal has been measured in the temperature range 4.2-290 K. Two minima in T-M(T) at 115 and 175 K were observed. The low-temperature T-M minimum has been identified as due to librational motions of the N2H5+ hydrazinium ion with an activation energy of 0.045 eV. The T-M minimum at 175 K has been assigned to rotation of the -NH3+ end of the hydrazinium ion with an activation energy of 0.074 eV. The activation energies have been compared with the results of a previous NMR and Raman spectroscopic studies of this crystal. (C) 2000 Elsevier Science B.V. All rights reserved.