We have used classical hot-atom chemistry to put tritium atoms inside fullerene molecules. The tritium is generated in a nuclear reactor with the reaction 6 Li(n, α) 3 H. The hot tritium atom slows down and can end up being thermalized inside a fullerene where it is trapped. The irradiated sample is dissolved, chromatographed, and counted in a scintillation counter, showing a small tritium activity. After some time, the sample is analyzed with a sensitive mass spectrometer, and 3 He was found on heating above 400°C, showing that the tritium had decayed leaving the 3 He trapped inside the fullerene.