Sulfate radicals, SO4.-, were generated using flash photolysis of aqueous S2O82- solutions 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 SO4.- radicals are observed at 100 mus 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 SO4.- radicals are adsorbed on the NP surface, leading to the formation of an adduct, with lambda(max) approximate to 320 nm (epsilon approximate to 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 SiO- sites with reaction rate constants of 1.5 x 10(14) x e(-(58+/-12)kJ/mol/RT) s(-1), < 10(3) x e(-(2+/-17)kJ/mol/RT) s(-1) and < 10(11) x e(-(46+/-3)kJ/mol/RT) s(-1), respectively. Two different SiO. surface defects, showing similar spectra (lambda(max) approximate to 600 nm) but different reactivities, are formed from the reaction of NP-sulfate radicals and deprotonated geminal and single silanols.