A novel technique has been employed to investigate the simultaneous damage to DNA components induced by soft X rays (1.5 keV) and low-energy electrons (0–30 eV) in thin films of thymidine deposited on glass and tantalum substrates and irradiated under atmospheric pressure and temperature. The films were surrounded by either an N₂ or O₂ environment. The formation of four radiation-induced products is reported in this article: base release, 5-hydroxymethyl-2′-deoxyuridine (5-HMdUrd), 5-formyl-2′-deoxyuridine (5-FordUrd) and 5,6-dihydrothymidine (5,6-DHThd). Analysis with LC-MS/MS shows larger damage yields in the samples deposited on tantalum than in those deposited on glass, which is attributed to the interaction of the additional low-energy electrons that are photoemitted from the metal surface. From a comparison of the results obtained from N₂ and O₂ environment, we report a dramatic effect from 6 O₂: an approximately threefold increase in the yield of products, attributed to the reaction of O₂ with initial carbon-centered thymidine radicals generated in the film during irradiation.