Synthetic duplex DNAs of repeating sequence, such as poly d(TTC).poly d(GAA), were separated into their individual single strands. The various single strands complexed not only, as expected, with their complementary strands, but also with other non-complementary strands. Characterization of such complexes with respect to stoichiometry, Tm values and the dependence of Tm on NaCl concentration showed that a variety of unusual structures could be inferred at physiological salt concentrations. These included extrahelical thymines, G.T oppositions, A.C oppositions and T.C oppositions.