Molecular Fulcher-a spectroscopy is used for the investigation of the HD and DT molecule formation in JET during H 2 and T 2 puffs, respectively. In the hydrogen case, the HD fraction in the outer divertor was about 10% of the molecular D 2 flux, resulting in a 1% contribution of H 2 to the D 2 flux using the law of mass action which is a very sensitive means to measure low quantities of H 2 or T 2 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 T 2 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.