Following fusion, 2′-deoxynucleosides undergo thermolytic cleavage to produce furfuryl alcohol and water along with the corresponding base. Contrary to previous suggestions, pyrimidine as well as purine deoxynucleosides are subject to base cleavage, the relative ease of thermolysis being 2′-deoxyguanosine > 2′-deoxyadenosine > 2′-deoxycytidine > 2′-deoxyuridine > thymidine. When dissolved in dimethyl sulfoxide, these nucleosides are susceptible to a similar cleavage at a lower temperature, with guanine being liberated from deoxyguanosine much more readily than adenine from deoxyadenosine, a feature which was in agreement with observations of Greer and Zamenhof (J.Mol.Biol.4, 123 (1962)) regarding thermal depurination of DNA.