A kinetic study of phase transition between phases I and II in ammonium chloride and ammonium bromide has been carried out using time-resolved powder diffraction (TRPD) and differential scanning calorimetry (DSC). The same transformation in ammonium iodide has been studied using DSC only. Reaction onset temperatures for NH4Cl, NH4Br and NH4I of about: 458 K, 412 K, and 253 K and enthalpies of transformation of: 4.4(2) kJ mol− 1, 3.3(1) kJ mol− 1 and 2.6(5) kJ mol− 1 were determined. The Avrami equation adequately described the shape of the degree of reaction curve for all three materials. For NH4Cl and NH4Br, the Avrami shape factor was equal to two. Taken in conjunction with previous microscopic studies, this implies very rapid nucleation followed by two-dimensional growth of the new phase. The value of the shape factor for NH4I switched from 2 to 4 about 55 s after the commencement of the transformation. Values of 50(20) kJ mol− 1 and 20(10) kJ mol− 1 were obtained for the activation energy before and after the switch. Activation energies for NH4Cl and NH4Br were found to be 400(80) kJ mol− 1 and 170(80) kJ mol− 1 from the DSC data. We conclude that for all three ammonium halides this transformation probably has the same mechanism.