Decauridylate containing exclusively a 2'-5' phospho-diester bond (2'-5'U₁₀) served as a template for the synthesis of oligoadenylates oligo(A)s from the 5'-phosphorimidazolide of 2'-5' diadenylate (ImpA-2'p5'A). Joining of 2'-5'U₁₀ and ImpA2'p5'A also took place in substantial amounts to yield long-chain oligoribonucleotides in the template-directed reaction. An unusual CD spectrum ascribed to helix formation between 2'-5'U₁₀ and 2'-5'(pA)₂ was observed under the same conditions as that of the template-directed reaction. The 3'-5' linked decauridylate (3'-5'U₁₀) also promoted the template-directed synthesis of oligo(A)s from ImpA2'p5'A, but more slowly compared with 2'-5'U₁₀. The results indicate that short-chain RNA oligomers with a 2'-5' phosphodiester bond could lead to longer oligoribonucleotides by template-directed chain elongation.