In numerous fields of application (environmental remediation, catalysis, nanomedicine), production ofhydroxyl radicals and solvated electrons by nanomaterials is a cornerstone. Through a very sensitive,nanoparticle-compatible, coumarin-based protocol, we quantified hydroxyl radicals in solution whenhydrogenated (H-ND) and oxidized (Ox-ND) detonation nanodiamonds were irradiated by MeV photons.We highlighted a blatant difference between the two surface chemistries as only H-ND led to 50% moreradicals, for irradiation doses and ND concentrations relevant in nanomedicine. For the first time, we alsoquantified solvated electrons after keV irradiation of both suspensions and showed that in the presenceof H-ND, hydroxyl radicals and solvated electrons were available in solution in equivalent and higheramounts than in water only. This asks the question of the mechanisms at stage and beside the negative/positive electron affinity hypothesis usually mentioned, we proposed, as for other nanomaterials, thatinterfacial water could play an essential role in radicals’ production in solution when detonation H-NDare irradiateD