[en] A method is described of preparing ¹⁴C-labelled 4-thiouracil and 4-thiouridine from ¹⁴C-labelled uracil and uridine using phosphorus sulfide. The thiation proceeds in an anhydrous dioxane medium. The advantage of the described method consists in the selective progress of the ¹⁴C-labelled uracil conversion reaction, and in the fact that as early as 45 mins of boiling the reaction mixture, the 4-thiouracil-2-¹⁴C yield is 96.6%. The dependence is shown of the reaction mixture composition on the reaction duration. The obtained results may be used in preparing ¹⁴C-labelled thio-derivatives of nucleotides and the appropriate bases, of which 4-thiouracil assumes the greatest importance in connection with the discovery of minority bases in nucleic acid chemistry. (J.B.)