Among the different nanodiamonds, milled nanodiamonds (MNDs) from high pressure high temperature (HPHT) synthesis exhibit a crystalline quality close to the bulk diamond, making it a serious candidate for quantum or energy-related fields. In this study, MNDs aqueous suspensions with oxidized (MND-Ox) and hydrogenated (MND-H) surface chemistries were irradiated with gamma-rays. Using optimized nanoparticle-compatible methods, we revealed that MNDs in suspension enhance the production of solvated electrons under irradiation, with a higher yield for MND-H. Also, for the first time, a surface chemistry-dependent scavenging effect of hydroxyl radicals was highlighted for this type of nanodiamonds.