In order to elucidate the mechanism of gas evolution in lithium-ion batteries, we fabricated carbon - LiNixCoyAl1-x-yO2 cells employing C-13-labeled ethylene carbonate (C-13-EC) and diethyl carbonate (C-13-DEC) as solvent components and then stored them at 85 C. The gas species evolved during storage tests were analyzed by gas chromatography/atomic emission detector to determine the isotopic ratio of CO2 and CO. The relative proportions of the CO2 derived from EC, DEC, and nonsolvent components were determined to be 52, 11, and 37%, respectively. The main source of CO2 was found to be EC. Further storage tests with either cathode or anode electrodes showed that the cathode components were a source of CO2, but anode components were not. As for evolved CO, the main source was found to be EC. Moreover, we also examined the gas-evolution behavior on the initial charge. The evolved gas species were mainly composed of H-2, C2H4, and CO. A minor amount of C2H6 was also detected. From our isotopic analysis it was shown that C2H4 was exclusively formed from EC, while C2H6 derived from DEC. In the case of CO, EC and nonsolvent components were found to be its sources. CO derived from DEC was not detected. (c) 2008 The Electrochemical Society.