The interaction of several aromatic cationic dyes such as, ethidium bromide (EB), methylene blue (MB), acridine orange (AO), and Hoechst 33258 with calf-thymus DNA and poly(A)-poly(U) duplex was investigated. The different induced extrinsic Cotton effects (greater than 300 nm) were observed for DNA- and RNA-dye complexes. The binding properties of these complexes were examined by UV, CD, and NMR spectroscopies.