AbstractResonance Raman spectra have been obtained at several uv wavelengths (200–266 nm) for poly(rA)‐poly(rU), poly(dA‐dU), poly(dA)‐poly(dT), and poly(dA‐dT), representing nucleic acid duplexes containing A‐U and A‐T base pairs with different stacking interactions and different backbone conformations. Frequency shifts are seen in the exocyclic modes corresponding to coupled C4O and C5C6 stretching of U and T, although the NH2scissors frequency of A is unshifted relative to that of the mononucleotide. These frequency patterns are interpreted as the superposition of H bonding and dipole–dipole coupling effects. Strong hypochromism is seen for most of the ring modes, resulting from the absorption hypochromism and from shifts in the electronic transitions due to stacking. The effects are larger for A bands in the homopolymers than in the heteropolymer duplexes, reflecting the larger influence of A/A than A/U(T) stacking. Poly(rA)‐poly(rU) stands out among these polymers in showing 10 cm−1downshifts in one U and several A modes. These shifts may be related to the A form structural parameters of this duplex, but the physical m