Comparisons have been made of the infrared absorption spectra of a few denatured sodium deoxyribonucleates (NaDNA) with that of native NaDNA. An absorption band has been observed at 1710 cm−1 of native NaDNA, but it is absent in any denatured NaDNA examined. This fact has been explained to be due to the breakdown of the specific hydrogen bonds between the base residues in the denatured NaDNA. The bands of native NaDNA at 1220, 1084 and 965 cm−1 have been assigned to the PO−2 antisymmetric stretching, PO−2 symmetric stretching, and P-O stretching vibrations, respectively. On the basis of the behaviors of these bands in breaking down the secondary structure of NaDNA, it has been concluded that a kind of “water of crystallization” is necessary for the formation of the double-helix structure of the DNA molecules.