Abstract: The present invention comprises a novel method of incorporating a tritium label at one or more predetermined sites within an oligonucleotide. In particular, the method comprises contacting a nascent, support-bound oligonucleotide having a free 5' hydroxyl group with a suitable oxidizing agent to oxidize the alcohol to an aldehyde, followed by reducing the aldehyde thereby formed with a suitable tritium labeled reducing agent such as ?.sup.3 H!NaBH.sub.4 to yield the 5' terminal alcohol with a 5' tritium label. Normal automated synthesis can then be continued to yield the oligonucleotide of desired length having the tritium label in the desired location. The oligonucleotides thereby produced have higher specific activity than those previously known in the art. According, in a second aspect, the present invention provides oligonucleotides having high specific acitivity.