The differences between thermal and vibrational properties of and are reported. The differential scanning calorimetry revealed that tetraamminenickel(II) perrhenate exhibits, in the temperature range of 300–140 K, one phase transition at ca. T c h = 188 K (on heating) and T c c = 185 K (on cooling). In the case of hexaamminenickel(II) perrhenate, no phase transition was observed in the same temperature region. Thermogravimetric measurements showed that the decomposition proceeds in the two main stages. In the first stage, a complete deammination takes place and next is released. TG measurements showed that molecules are not equivalently bonded to central atom. The final and intermediate products of decomposition were analysed by means of infrared spectroscopy. The final product of thermal decomposition of both compounds is nickel(II) oxide. The analysis of far infrared spectra revealed that anions in have disturbed tetrahedral symmetry, whereas in they seem to form polymeric chains. The variation of the activation energies of the deammination and decomposition steps of was calculated through the model-free isoconversional Kissinger–Akahira–Sunose method and model-free Kissinger method. Infrared spectra were calculated by the DFT method and quite a good agreement with the experimental data was obtained.