Protected phosphoramidite 2 was used to incorporate synthetically the non-natural nucleoside 4-thio-2’-deoxyuridine (d(S4)U) at a specified position within oligonucleotide 3. Following DNA synthesis, the thiocarbonyl group of the 4-thio-2’-deoxyuridine of 3 was modified chemoselectively and quantitatively with thiol-specific reagents to afford oligonucleotides 14, 15, and 16 by S-alkylation or mixed disulfide formation. This protocol represents a divergent postsynthetic modification method for incorporating a wide range of functional groups at any base position within a DNA strand, starting from a single oligonucleotide intermediate.