Adenine, guanine, thymine, cytosine, uracil, their nucleosides and nucleotides were treated with ozone at pH 5, 7 or 9 and their absorption spectra were estimated at different ozonation time. Except adenine and adenosine at pH 5, the spectra varied depending on pH, indicating the reaction with ozone. Some differences in the variation were also observed among bases, nucleosides and nucleotides.