Infrared spectra were used to show that the sodium salts of acetate, sulfate and phosphate (pH 7.2) selectively stabilize some of the alternative structures of poly(dG-dC).Na and poly(dA-dT).Na as a function of hydration in nonoriented gels. NaCl was used as a reference. Each anion was present at 0.36 mole per mole of nucleotide residue. The weak absorption bands from these anions did not interfere with conclusive interpretation of the IR spectra of the polynucleotides. Poly(dG-dC).Na assumed the usual B* structure with each of the anions at high hydrations (r.h. of the ambient air > or = 94%). Lowering the hydration gave the following results. With acetate, the B* structure remained with only a small fraction of a modified Z or some other unusual structure present. With sulfate or phosphate, a sharp transition to the Z structure occurred (essentially complete by 86% r.h.). With reference to chloride ions, acetate favors the B* while sulfate and phosphate (pH 7.2) favors the Z structure. Poly(dA-dT).Na assumed the usual B structure with each of the anions at high hydrations. Lowering the hydration gave the following results. With acetate, the A structure was observed at the same hydrations as with chloride. With sulfate, a sharper transition to the A structure occurred (complete by 80% r.h.). With phosphate, a still sharper transition to the A structure occurred (complete by 86% r.h.). With reference to chloride, acetate shows little difference but sulfate and phosphate (pH 7.2) promote the A over the B structure. These results are compared with past results for NaNO3.