Synthesis of (¹⁵N₂)[¹⁷O]Urea, (¹⁵N₂)[O²,O⁴-¹⁷O₂]Uridine and (¹⁵N₃)[O²-¹⁷O]Cytidine   A general synthetic approach for the synthesis of ¹⁵N- and ¹⁷O-doubly labelled pyrimidine nucleosides is described. The ¹⁵N isotopes in uridine and the ¹⁷O isotope in the urea-derived carbonyl group of uridine and cytidine originate from (¹⁵N₂)[¹⁷O]urea which was synthesized from ¹⁵NH₄Cl, thiophosgene, and H₂[¹⁷O]. The third ¹⁵N isotope of cytidine in 4-position stems from the substitution of the 1,2,4-triazole moiety of the (¹⁵N₂)[O²-¹⁷O]uridine derivative with ¹⁵NH₄OH. Hydrolysis of the same key intermediate with Na[¹⁷O]H/H₂[¹⁷O] introduced the second ¹⁷O isotope into the 4-position of uridine. The ¹⁵N- and ¹⁷O-NMR spectra of the target compounds in phosphate-buffered H₂O serve as references for heteronuclear NMR spectra of labelled RNA fragments.