EPR spectroscopy was used to investigate the influence of cysteamine on free radical formation in the DNA nucleotide dCMP induced by ionizing irradiation in a low temperature aqueous environment. Both frozen aqueous solutions (both H₂O and D₂O) and BeF₂ glasses (H₂O and D₂O) were applied. In the absence of cysteamine, the spectra obtained at 77 K are a superposition of signals of ·OH (or ·OD) radicals, of one-electron reduced cytosine base radicals and of radicals containing an unpaired electron at the C1' site of the deoxyribosephosphate. Whereas the formation of the latter is completed at 77 K in the frozen aqueous solutions, its yield grows between 77 K and 140 K in the BeF₂ glasses. In both cases the final concentration of the sugar radical comes to about 20 to 25% of all radicals of the nucleotide. In the presence of cysteamine, no sugar radicals at all are found in the frozen aqueous solutions and their formation is diminished in the BeF₂ glasses at 77 K. In addition, spectra of thiyl and perthiyl radicals could be detected. After removal of the pattern of the thiyl radical in the frozen H₂O solution, the spectrum of the one-electron reduced cytosine species can be isolated as a triplet with a total spread of about 2.6 mT. Cysteamine has little influence on the secondary reactions. A reaction with the (protonated) 'anions' in the frozen aqueous is suggested by their early decay in presence of the thiol. In the BeF₂ glasses, both C1' species and thiyl radicals are increased in concentration from reactions with the decaying ·OH radicals upon annealing.