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Due to the high amount of water in biological tissue, most of the inelastic scattering processes between the incoming high-energy radiation ( γ) and tissue occur with the solvent. To improve therapeutic outcome and to develop more effective radiosensitizers, a better understanding of the underlying damage mechanisms is necessary 1, 2, 3. The most important types of damage, which can lead to genetic instability, are single strand breaks (SSB) and double strand breaks (DSB) at the sugar–phosphate backbone and the loss or chemical modifications of the nucleobases. In isolated DNA molecules, the damage can occur at its different building blocks, the sugar–phosphate backbone, and the nucleobases. Hereby, DNA damage is of key interest due to its central role in reproduction and mutation. The damage to biomolecules caused by ionizing radiation is the reason behind treating of cancer via radiation therapy 1. In consequence, base damage and base release become predominant, even though the number of strand-breaks increases further. In contrast, a strong increase of DNA damage is observed in water, where OH-radicals are produced. The exposure of dry DNA to x-rays leads to strand-breaks at the sugar-phosphate backbone, while deoxyribose and nucleobases are less affected. The results allow us to distinguish direct damage, by photons and secondary low-energy electrons (LEE), from damage by hydroxyl radicals or hydration induced modifications of damage pathways. They permit in-situ monitoring of the effects of radicals on fully hydrated double-stranded DNA. Here we present near-ambient-pressure XPS experiments to directly measure DNA damage under water atmosphere. Although water radiolysis is essential for radiation damage, all previous XPS studies were performed in vacuum. X-ray photoelectron-spectroscopy (XPS) allows simultaneous irradiation and damage monitoring. Ionizing radiation damage to DNA plays a fundamental role in cancer therapy.