AbstractThe alternative structures of the lithium, sodium, potassium, and rubidium salts of poly(dA‐dT) were studied using ir spectra of hydrated, nonoriented gels. Poly (dA‐dT) · Na existed as the B structure at high hydration and as the A structure at moderate hydration. A disordered state occurred at low hydration. No hint of the C, D, or other structures was observed at any hydration for nonoriented gels containing NaCl up to 0.56 moles NaCl/mole of nucleotide residue. Poly (dA‐dT) · Li existed as the B structure at high hydration, made a gradual transition to the C structure at moderate hydration, and became disordered at low hydration. Poly (dA‐dT) · K and poly (dA‐dT) · Rb existed as the B structure at high hydration and as the presumptive D structure at lower hydration. The addition of Zn(II) to poly (dA‐dT) · Na produced a gradual transition from the B to the C structure upon dehydration. The addition of NaNO3 to poly (dA‐dT) · Na gave only the A and B structures, and not the C or Z structures that are promoted by NO3 in other base sequences. The DNA secondary structure stabilized by a given ion is determined by a specific base sequence and not simply by the properties of the ion.