Abstract Recent results obtained by dilatometric, calorimetric, X-ray, and NMR methods for the phase transitions in ammonium and deutero-ammonium halides are reported. The dilatometric observations on the kinetics of the transitions I-II of NH4Cl and NH4Br, and the effects of deuteration on the I-II transformation temperature and volume change, and on the thermal expansion coefficients of the modifications I and II of NH4Br are described. Lattice parameter data, obtained by X-ray diffraction for NH4Cl between 167° and 250°. for NH4Br and ND4Br between -192° and 196 °C, and for NH4I between -116° and +22 °C are presented. From these data, the molar volume changes for the I-II transition of NH4Cl, for the I-II, II-III, and III-IV transitions of NH4Br and ND4Br, and for the I-II and II-III transitions of NH4I are reported. The effect of deuteration on the thermal expansion of modifications I, II, and III of NH4Br, and the structural nature of the III-IV transition of NH4Br are examined. The energies of the t...