This paper describes the chemical characterization and helix-forming properties of a series of deoxyoligonucleotides with the repeating, self-complementary base sequence ATAT.... Their preparation has been described elsewhere (Elson & Jovin, 1968). Two enzymic methods show that the nucleotide at the 5′-end of each oligomer chain is T, so that the generic structure of the oligomers is d(pTpA)t. Their molecular weights have been determined with the ultracentrifuge, with the aid of a sample of known molecular weight synthesized in the laboratory of H. G. Khorana. At low counterion concentrations (< 0.01 m-Na+), d(TA)t oligomers with i > 7 form one-chain, hairpin helices. These show simple monophasic melting curves. At intermediate or high counterion concentrations, two-chain helices are formed and these show biphasic melting curves. Molecular weight measurements demonstrate that the two-chain helices are first converted into one-chain helices at low temperatures, and then these melt at higher temperatures. On cooling in these conditions, a slow reaction is observed at low temperatures which appears to be the conversion of hairpin to straight-chain helices by chain slippage.