Abstract Nanoparticles devoted to improve radiotherapy treatments are an efficient tool if they can induce the formation of deleterious species in the tumor. Their interaction with radiation is responsible for radical production but in spite of the numerous studies mostly with cells, no consensus has been reached about radical formation mechanism. In order to gain knowledge in the physico-chemical step of this phenomenon, we applied a very sensitive test to quantify hydroxyl radicals and electrons produced when gold atoms, organized as nanoparticles or as a salt in solution, are irradiated by keV and MeV photons (x- and γ - rays). The crucial role of interfacial water is suggested to explain the high quantity of radicals measured for nanoparticles. These experimental data were supplemented by classical molecular dynamics simulations, revealing a specific organization of the water hydrogen bonding network at the nanoparticle surface which could be a key component in the mechanism of radical production by irradiated colloidal suspensions.