Abstract Titration studies on a series of oligo (A)'s are described and the thermal stability of the structural form at acid pH has been examined. The stability of complexes formed between oligonucleotides and a complementary polynucleotide has also been studied using homologous series of oligo (A)'s, (C)'s, (I)'s, and (U)'s, and a wide variety of polynucleotides including polynucleotide analogues. Up to oligonucleotide chain lengths of about 12 plots of log n against 1 T give straight lines. For a given oligonucleotide series complexed with a variety of complementary polynucleotides, such lines are parallel. Although independent of the polynucleotide, the slope is characteristic of the oligonucleotide and differs with the nature of the base. Thus the Tm for oligo (A)n · poly (U) differs from that of oligo (U)n · poly (A) as does the rate of increase of stability with chain length. After chain lengths corresponding to one turn of the double helix, a second function between chain length and Tm appears but the slopes are no longer parallel for a given oligonucleotide series complexed with different polynucleotides. Displacement reactions are described in which the product complex has a Tm lower than that of an original secondary structure present in one of the reactants.