A study of the thermal decomposition of tetrahydroxy nickel carbonate NiCO3. 2Ni(OH)2.4H2O, was performed by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) under non-isothermal conditions in dynamic atmospheres of dry air or nitrogen, at different heating rates. Non-isothermal kinetic and thermodynamic parameters were calculated from the effect of heating rates on the maximum temperature of TG and DTA peaks, as well as from the recorded DSC data, using the Ozawa method. The parent material, the intermediates and the final product were characterized by means of infrared spectroscopy and X-ray diffractometry (XRD). The decomposition of tetrahydroxy nickel carbonate commenced through two steps (dehydration and decomposition). The dehydrated intermediate from the first step was found to contain an XRD detectable NiOOH phase. The morphological changes throughout the decomposition course were examined by scanning electron microscopy (SEM) and found to be in accord with the thermal analysis. A reaction pathway is proposed.