Ultaviolet absorption spectra down to 185 mμ are presented for a number of nucleic acids and synthetic polynucleotides in the helical and random forms, and of their constituent mononucleotides, as well as the corresponding nucleosides and free bases. A short wavelength absorption band is in all cases observed with a maximum varying between about 187 and 201 mμ. These bands are more intense than those near 260 mμ. The helical polymers show a hypochromic effect in this region of a somewhat similar magnitude to that in the 260 mμ region. Other spectral phenomena accompanying denaturation and degradation are described.