The coupling of 2-cyanoethyl thymidine phosphoramidite to solid-support-bound, phosphate-unprotected oligothymidylates and their phosphorothioate analogues was studied. The yield of the coupling reaction depended on the pK(BH)()+ values of protonated nitrogen bases that served as counterions to the phosphodiester functions of oligonucleotides. To maximize the coupling efficiency, the oligonucleotides were detritylated and washed with a mixture of 0.1 M DMAP and 0.1 M 1H-tetrazole, which resulted in a 98+% coupling efficiency. The utility of the results was demonstrated in the preparation of oligonucleotides with a mixed backbone that required the successive use of H-phosphonate and phosphoramidite methods of synthesis. Using this approach, 20-mer antisense oligonucleotides containing 2'-O-(2-methoxyethyl) ribonucleoside residues and phosphorothioate and phosphoramidate internucleosidic linkages were synthesized in high yield.