Lithium nickelate is a promising electrode material for lithium-ion batteries, however, its low thermal stability is a problem that should be solved. We studied the thermal stability of delithiated lithium nickelate compounds in the presence of electrolyte solvents/solutions. We measured the exothermal heat generation of the reaction system using differential scanning calorimetry. We examined simple systems such as ethylene carbonate with Li0.2NiO2, using hermetically sealed pans that allowed us to perform the experiments under constant mass conditions. The data were quantitatively analyzed to propose possible reaction mechanisms for the exothermal behavior. We attributed the main source of the exothermal heat to organic solvent combustion caused by oxygen released from the nickelate. We studied similar reactions using other solvents/solutions and electrochemically or chemically delithiated nickelate samples. We also examined the behavior of lithium cobaltate for comparison. We discuss the effect of partial substitution for nickel on thermal stability.