Abstract: The use of the combination of gas chromatography (GC) and gas chroma-tography-mass spectrometry (GCMS) for the determination of hydrogen,deuterium, tritium, helium, oxygen, nitrogen and carbon monoxide in gas mixtures is described. The precision and accuracy is 1.5-2.0% for a 10ml sample of the gas-mixture. The ratios of the atomic ions to molecular ions in the HD, HT and DT molecule can be obtained by GCMS. It is found that, the ratios of H+/HD+and D+/HD+,H/HT+ and T+/HT+,D+/DT+ and T+/DT+ are unequal, and that the separation characteristic of HT and D2 is poor when CO is present in the gas mixture.