The copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a highly effective method for the selective incorporation of deuterium atom into the C-5 position of the 1,2,3-triazole structure. Reactions of alkynes and azides can be conveniently carried out in a biphasic medium of CH(2)Cl(2)/D(2)O, using the CuSO(4)/Na ascorbate system. The mildness of the method renders it applicable to substrates of relatively high complexity, such as nucleosides. Good yields and high levels of deuterium incorporation were observed. A reaction conducted in equimolar H(2)O and D(2)O showed 2.7 times greater incorporation of hydrogen atom as compared to deuterium. This is consistent with the H(+) and D(+) ion concentrations in H(2)O and D(2)O, respectively. With appropriately deuterated precursors, partially to fully deuterated triazoles were assembled where the final deuterium atom was incorporated in the triazole-forming step.