AbstractThe labelling of TX and TG with35S has been performed by Moravek and Nejedly (1), by isotopic exchange with elemental radiosulphur in boiling pyridine solution. The authors do not give any indications regarding the kinetics and the exchange yields. 2‐Thio‐uracil has been labelled only by synthesis in 1958 by Ziegler (2).We have done this study, aiming to find some simpler labelling methods than those described in the literat