In this study, reactive free radicals derived from several nucleosides were spin trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and then detected by high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry (HPLC/ESI-MS/MS). This method provides a specific detection of spin trapping adducts derived from nucleosides with a very high sensitivity: quantities as low as 0.5 picomoles of spin trapping adducts corresponding to concentrations of 2.5 10(-8) mol . L(-1) were detected. Different spin trapping adducts were characterized by HPLC/ESI-MS/MS in three well-known systems producing free radicals photochemically: the photolysis of 5-halo-2'-deoxyuridines, the photolysis of 5-thiophenylmethyl-2'-deoxyuridine and the photolysis of thymidine with menadione bisulfite as a photosensitizer. A new radical photoreactivity of uridine derivatives was also detected by this method both at the nucleoside and at the RNA level, showing that the method is also relevant for studying spin trapping adducts derived from DNA and RNA strands.