Detailed investigation on the thermal behaviour of hexaamminenickel(II) sulphate has been carried out by TG/DTA coupled online with mass spectroscopy (MS) and temperature resolved X-ray diffraction (TR-XRD). Simultaneous TG/DTA coupled with MS studies in helium atmosphere detected the presence of NH, N, S, O and N2 fragments during thermal decomposition. The complex undergoes a four stage decomposition pattern. The kinetics of the thermal decompositions was studied using model-free isoconversional methods viz ., Friedman, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) equations. Isoconversional methods yielded a series of activation energy which varies with the extent of conversion, indicating the complex nature of solid state reactions. The intermediates formed were detected by TR-XRD. The morphology of the intermediates was studied by scanning electron microscopy (SEM). The final residue was found to be nickel oxide in the nano range.