AbstractThe presence and assignment of low frequency infrared bands characteristic of the A, B and C forms of DNA (Salmon Sperm and Micrococcus Lysodeikticus) and of synthetic poly (dA‐T): (dA‐T) and of polyribonucleotides‐A form are considered. The vibrational modes that may arise from the phosphate diester stretch of DNA backbone are investigated in the region of 750 cm−1–900 cm−1. The proposed assignment of the IR bands occurring at about 837 cm−1 to the vibrations of the phosphate diester C–O–P–O–C chain in a specific conformation allow characterization of the backbone geometry in the B and C forms. The bands situated at 812 cm−1 and 860 cm−1 are undoubtedly characteristic of the backbone phosphodiester chain of the A form. The possibility of other contributions to the 812 cm−1 band is also discussed.