Both tritium and deuterium will be used in equal measure as fuel species in future fusion reactors. A number of separate experiments have been conducted at JET over the years in which tritium has been introduced in various quantities relative to the deuterium content, the most recent of which was the "trace tritium" campaign, in which tritium has been puffed both from the wall and injected via the neutral beam heating line. The quantity of injected tritium was maintained at very low levels (less than 1%) in order to keep the radiation level low and decontamination time of the vessel short. Since gas puffed tritium is introduced as a molecule via the nozzle and recycles mainly in the form of hydrogen isotopomeres, it is of interest to investigate the formation, particularly of DT molecules, with regard to the total fuelling of T achieved in the discharge. This contribution describes the use of molecular spectroscopy, employed in a similar way to that reported in, to investigate DT molecule formation for the first time in JET.