Abstract The temperature dependence of 2H NMR spectra and 81Br and 127I NQR frequencies was investigated for the deuterated compounds of [C(NH2)3]HgBr3 and [C(NH2)3]HgI3 between 77 and ca. 400 K. Due to deuteration, the phase-transition temperatures of the iodine compound were increased by 2 and 6 K for the second-order (251 K) and the first-order (210 K) phase transitions, respectively. Distinctive 127I NQR frequency-shifts of more than 200 kHz were observed in the intermediate phase of the iodine compound on deuteration. The temperature dependences of the 2H NMR spectra for [C(ND2)3]HgBr3 and [C(ND2)3]HgI3 could be explained by invoking C3 reorientation of the guanidinium cation. The activation energies for the reorientation evaluated from 2H NMR were consistent with the former 1H NMR T1 results with the exception of the low-temperature phase of the iodine compound. The quite low activation energy from 2H NMR spectrum for this phase may indicate that the quenching of the orientation disordering of the cations is responsible for the fade-out phenomenon in the 127I NQR resonance lines below ca. 100 K.