The reactivity of sodium inserted hard carbon with ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), EC:DEC, EC:DMC and NaPF6 EC:DEC (/DMC) electrolytes was studied by Accelerating Rate Calorimetry (ARC), and the products after the ARC experiments were investigated by X-ray diffraction. The results show that sodium inserted hard carbon reacts with DMC to form sodium methyl carbonate, and that it reacts with EC and DEC to form sodium alkyl carbonates which apparently have a similar structure to sodium methyl carbonate. Sodium inserted hard carbon is more reactive with DMC and DEC than with EC. Finally, Na inserted hard carbon is more reactive in NaPF6 EC:DEC and NaPF6 EC:DMC than in EC:DEC and EC:DMC due to the high thermal stability of NaPF6 and preferential solvation of NaPF6 by EC, which leaves DEC and DMC available for reaction. The reactivity of Li inserted hard carbon was also compared to the reactivity of Na inserted hard carbon in both solvent and electrolyte and it was found that Li inserted hard carbon shows much better thermal stability. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.jes111637] All rights reserved.