Some optical and thermodynamic properties of the double helical form of the alternating copolymer poly(A‐U) and the two stranded complex of poly A and poly U are compared. It is shown that their thermal difference spectra are closely similar when that of the latter complex is corrected for the hypochromicity of the poly A formed on denaturation. Similarly differences in their Tm and pHm values can be accounted for in terms of the free energy of stacking of the poly A formed on denaturation. It is suggested that sequence effects are of only marginal consequence in determining the optical and thermodynamic properties of two stranded helical polynucleotides.