2’,5’-Linked DNA oligomers bearing the bases guanine and cytosine are found to associate. Three 2’,5’-DNA strands composed entirely of the bases guanine and cytosine, two mutually complementary dodecamers, and a self-complementary octamer were prepared. Melting temperatures and free energies of duplex formation were determined for these oligomers and their natural, 3’,5’-linked DNA counterparts. From these data it was seen that 2’,5’-DNA duplexes have free energies approximately half that of the corresponding natural DNA duplexes under high ionic strength conditions (1 M NaCl). 2’,5’-DNA strand association was also found to persist under conditions of low solution ionic strength. These observations are consistent with 2’,5’-linked DNA comprising an alternative genetic material. A model is proposed where present-day 3’,5’-linked nucleotides arose by "genetic takeover" of 2’,5’-linked nucleotides.