Oligonucleotides carrying 2'-aldehyde groups were synthesized and coupled to peptides containing an N-terminal cysteine, aminooxy or hydrazide group to give peptide-oligonucleotide conjugates in good yield. The synthesis of a novel phosphoramidite reagent for the incorporation of 2'-O-(2,3-diaminopropyl)uridine into oligonucleotides was also described. Resultant 2'-diaminooligonucleotides may be useful intermediates in further peptide conjugation studies.