The degradation of nucleobases, ribonucleosides and ribonucleotides with O3 was examined as a model of the action of O3 on nucleic acids. The degradation rates of nucleobases were in the following order: G,T > U > C > A. Nucleosides and nucleotides were degraded in the same order as in the corresponding nucleobases. The ribose moiety was degraded more slowly than the base portion in nucleosides and nucleotides except in the cases of adenosine and AMP. The degradation of D-ribose and D-ribose-5-phosphoric acid (R5P) was also examined, and it was found that R5P resisted the attack of O3. Pi was liberated following degradation of the ribose moiety. These findings have relevance to the mechanism of the disinfectant and mutagenic activities of O3.