Purpose: The main aim of the present study is to gain mechanistic ă insights into the modulating effect of molecular hydrogen on the ă -radiation-induced alteration pathways of DNA nucleobases.Materials and ă methods: Aerated aqueous solutions of calf thymus DNA were exposed to a ă Co-60 source at doses ranging from 0 to 55Gy under normoxic conditions, ă in the presence or not of 0.7MPa hydrogen or helium. The measurement of ă several modified bases was performed using HPLC associated with ă electrospray ionization tandem pass spectrometry (HPLC-ESI-MS/MS). ă Bleaching of aqueous solutions of p-nitrosodimethylaniline (p-NDA) ă solutions was also used to allow the quantification of hydroxyl radical ă (center dot OH) formation.Results:pNDA bleaching was significantly ă reduced in the presence of hyperbaric hydrogen. This is undoubtedly due ă to (OH)-O-center dot scavenging by H-2 since, under the same conditions, ă He had no effect. Similarly, base alterations were significantly reduced ă in the presence of hydrogen, as compared to controls under normal ă atmosphere or in the presence of helium. The relative proportions of ă modified nucleobases were not changed, showing that the only effect of ă H-2 is to scavenge (OH)-O-center dot without exhibiting reducing ă properties.Conclusions: Our findings demonstrate that H-2 exerts a ă significant protection against radiation-induced DNA base damage in ă aqueous solutions, (OH)-O-center dot scavenging being the only mechanism ă involved.