The paper describes the synthesis, characterization and thermal decomposition of nickel(II) bis(tartrato) nickelate(II) heptahydrate [Ni2(C4H4O6)2]·7H2O. The complex was characterized by elemental analysis, magnetic moment measurement, infrared, ESR and electronic spectroscopy. The experimental evidences indicate that complex is likely to have metal bonding. The thermal decomposition of the complex produced NiO in air at about 360°C and in nitrogen at about 380°C as the final product. Some of the intermediates produced during the thermolysis were isolated by temperature arrest technique and identified by analytical and spectroscopic methods. A tentative reaction mechanism is proposed for the thermal decomposition of the complex in air and nitrogen.