Rapid synthesis of radiolabeled pyrimidine nucleosides or nucleotides from stannyl precursors

A method of making a radiolabeled pyrimidine nucleoside or nucleotide is described. In the method, a stannylated pyrimidine nucleoside or nucleotide is contacted in an aqueous solution with a radioactive iodide, bromide, chlorine or astatine ion in the presence of an acidic hydrogen peroxide oxidizing solution comprising at least a 3:1 ratio of 30% hydrogen peroxide to 1N acid (v/v), whereby a water soluble pyrimidine nucleoside or nucleotide labeled with radioactive iodine, bromine, chlorine or astatine is formed. Kits suitable for practicing the method are also disclosed.