LiCoO2, LiNiO2 and LiMn2O4 are all stable in air to high temperatures. By contrast, LixCoO2, LixNiO2 and LixMn2O4 (x<1) are metastable and liberate oxygen when they are heated in air or in inert gas. The temperature at which oxygen evolution occurs depends on x and on the material. Using thermal gravimetric analysis and mass spectrometry, we have studied the thermal decomposition of these materials in inert gas. We find that the nickel materials are least stable, the manganese compounds are most stable, and that the cobalt compounds show intermediate behaviour. These results have important consequences for the safety of Li-ion cells, and suggest that cells using LiMn2O4 as the cathode should be safer than those using LiNiO2 or LiCoO2.