Thermal stability of Fully charged 550 mAh prismatic Li-ion cells (Sn-doped LiCoO2/graphitic carbon) and their components are investigated. Accelerating rate calorimetry (ARC) is used to determine the onset temperature of exothermic chemical reactions that force the cell into thermal runaway. Differential scanning calorimetry (DSC) and thermogravimetry anslysis are used to determine the thermal stability of the cell's positive electrode (PE) and negative electrode (NE) materials from 35 to 400 degrees C. The cell self-heating exothermic reactions start at 123 degrees C, and thermal runaway occurs near 167 degrees C. The total exothermic heat generation of the NE and PE materials are 697 and 407 J/g, respectively. Heat generations of the NE and PE materials, washed in diethyl carbonate (DEC) and dried at approximate to 65 degrees C under vacuum, are significantly lower than unwashed samples. Lithium plating increases the heat generation of the NE material at temperatures near the lithium melting point. Comparison of the heat generation profiles from DSC and ARC tests indicates that thermal runaway of this cell is close to the decomposition temperature range of the unwashed PE material. We conclude that the heat generation from the decomposition of PE material and reaction of that with electrolyte initiates thermal runaway in a Li-ion cell, under thermally or abusive conditions.