Molecular Fulcher-a spectroscopy is used for the investigation of the HD and DT molecule formation in JET during H2 and T2 puffs respectively. In the hydrogen case, the HD fraction in the outer divertor was about 10% of the molecular D2 flux, resulting in a 1% contribution of H2 to the D2 flux using the law of mass action which is a very sensitive means to measure low quantities of H2 or T2 respectively via the detection of the isotopomeres HD and DT. Since the puffed tritium was about one order of magnitude smaller than the H flux introduced, a DT-flux of only ~1% is expected, resulting in a T2 flux of 0.1%. Evidence will be presented that similar levels have indeed been detected, although the signals were near the detection limit, demonstrating that this type of spectroscopy offers an interesting potential tool as a fuelling and recycling rate diagnostic.