5-Methylcytosine is a minor nucleobase of eukaryotic DNA which plays a central role in the regulation of gene expression. UV-A irradiation of an aerated aqueous solution of 5-methyl-2’-deoxycytidine in the presence of menadione (MQ) as a type I photosensitizer leads to the formation of several stable oxidation products. Emphasis was placed in this study on the isolation and the characterization of the major class of decomposition products whose formation involves the oxidation of the methyl group. These include 5-(hydroperoxymethyl)-2’-deoxycytidine and two stable decomposition products namely, 5-(hydroxymethyl)-2’-deoxycytidine and 5-formyl-2’-deoxycytidine. Structural assignment df the latter modified nucleosides was inferred from extensive spectroscopic measurements (H-1 and C-13 NMR, UV spectroscopy, and mass spectrometry). In addition, conformational analysis of the oxidized nucleosides was inferred from detailed H-1 NMR analysis. All of the above photooxidation products appear to arise from the deprotonation of the 5-methyl-2’-deoxycytidine radical cation which is generated by menadione photosensitization.