Polypurine DNAs and RNAs containing at least 33% guanine form a stable secondary structure at neutral pH and moderate ionic strengths. The tm's of the polymers increase with increasing guanine content. To eliminate possible structures three novel polymers, d(Gn2A)n, d(Gm6A)n and d(IA)n, as well as the random copolymer rr(G,A)n were were studied. Both d(Gn2A)n and d(IA)n can form a secondary structure whereas d(Gm6A)n and r(G,A)n cannot. Model building suggested two possible structures, one a duplex and the other a tetra-stranded polymer. The latter is considered to be the more likely, since previous X-ray diffraction studies have shown that rGn and rIn are tetra-stranded. Circular dichroism spectra are also consistent with such an interpretation.