Sulfate radicals, SO 4 •-, were generated using flash photolysis of aqueous S 2 O 8 2solutions and the reactions of the inorganic radicals with the surface of suspended silica nanoparticles (NP) investigated. In the presence of colloidal silica no absorption traces due to SO 4 •radicals are observed at 100 µs after the flash of light. However, two transient species with absorption maxima around 320 and 600 nm are formed. A kinetic analysis of the experimental results indicate that SO 4 •radicals are adsorbed on the NP surface, leading to the formation of an adduct, with λ max ≈ 320 nm ( ≈ 7000 cm -1 M -1 ), and showing similar reactivity to that observed for the sulfate radical in aqueous solutions. The NP-sulfate radical adducts react with adsorbed water, and with single and geminal SiOsites with reaction rate constants of 1.5 × 10 14 × e -(58(12)kJ/mol)/RT s -1 , < 10 3 × e -(2(17)kJ/mol/RT s -1 and < 10 11 × e -(46(13)kJ/mol/RT s -1 , respectively. Two different SiO • surface defects, showing similar spectra (λ max ≈ 600 nm) but different reactivities, are formed from the reaction of NP-sulfate radicals and deprotonated geminal and single silanols.