[en] Complete text of publication follows. Aerated aqueous solutions of the nucleotide 5'-guanosine monophosphate (5'-GMPNa₂) have been irradiated with ⁶⁰Co γ-rays. The yield of the disappearance G(-5'-GMPNa₂) determined by measuring the absorbance at 254 nm, was found equal to 0.3. On the other hand deaerated solutions in the presence of Mg²⁺ ions or solutions saturated with N₂O, presented a double G-value equal to 0.6. Further experiments showed that Mg²⁺ ions do not affect the G(-5'-GMPNa₂) in irradiated aerated solutions, which was found to be 0.4. These results can be interpreted by considering the radiolysis products of the water, as well as the standard reduction potential of the pair Mg²⁺/Mg and that of the radicals O₂^.- and e_aq^-