The thermal decompositions of polycrystalline samples of were studied by thermo- gravimetric analysis with simultaneous gaseous products of the decomposition identified by a quadruple mass spec- trometer. Two measurements were made for samples placed in alumina crucibles, heated from 303 K up to 773 K in the flow (80 cm 3 min

1 ) of Ar 6.0 and He 5.0, at a constant heating rate of 10 K min

1 . Thermal decomposition process undergoes two main stages. First, the deamination of [Ni(NH 3 ) 6 ](NO 3 ) 2 to [Ni(NH 3 ) 2 ](NO 3 ) 2 occurs in four steps, and 4NH 3 molecules per formula unit are liberated. Then, decomposition of survivor [Ni(NH 3 ) 2 ](NO 3 ) 2 undergoes directly to the final decomposition products: NiO 1 ? x ,N 2 ,O 2 , nitrogen oxides and H 2 O, without the formation of a stable Ni(NO 3 ) 2 , because of the autocatalytic effect of the formed NiO 1 ? x . Obtained results were compared both with those published by us earlier, by Farhadi and Roostaei-Zaniyani later and also with the results published by Rejitha et al. quite recently. In contradiction to these last ones, in the first and second cases agreement between the results was obtained.