A facile method for the synthesis of highly enriched ¹⁸O labeled pyrimidine ribonucleosides is described. The ribonucleoside may be labelled specifically in the base, the sugar, or both moieties with one or two oxygen-18 atoms. The isotopic purity of the products as well as the location of the oxygen-18 labels have been unambiguously determined by mass spectrometry. Stable isotope labeled analogs have been employed to determine the composition of several clinically significant nucleoside dialdehydes by mass spectrometry. Formation of the trimethylsilyl derivatives permits the gas chromatographic separation of the major components present in the equilibrium mixture. In addition to the expected hemiacetals and hydrates, a substantial amount of the dialdehydes exist in polymeric form. Fast atom bombardment mass spectrometry enabled observation of dimeric and trimeric species from the polymeric material present in the mixture.