Noble gas atoms can be introduced into the interior of fullerene molecules. A procedure, using a mass spectrometer system, is described for measuring the rates and amounts of noble gas released on heating these compounds. We find that the half life for Ne@C-60 reacting to Ne+C-60 is at least many weeks at 630 degrees C. Additionally, the rate of thermal decomposition of fullerenes can be very substantially faster if traces oi trapped solvent are not removed. (C) 1996 Elsevier Science Ltd