The preparative Gas Chromatographic (GC) system at JET was designed to produce deuterium and tritium of purity > 98%. Displacement gas chromatography with Pd as the packing material is used. The paper explains briefly this batch method and explains in detail the tritium commissioning as well as the full use and performance of the GC system during the Deuterium-Tritium Phase and the subsequent clean-up phase at JET. More than 160 g of tritium were handled in 160 GC isotope separation runs. The best tritium and deuterium qualities achieved were in the range of 99.9 and 99.7%, respectively. Finally, a few possible improvements of the GC system are discussed.