The reduction of methyl viologen ions, MV2+, adsorbed on SiO2 nanoparticles in N2O saturated colloidal suspensions has been studied using the pulse radiolysis technique. Reduced methyl viologen, MV+, was produced within the electron pulse, followed by slower reactions of MV2+ with OH and H radicals. The fast formation of MV+ was taken as evidence that electrons produced in the initial ionization events within the SiO2 particles rapidly migrate to the surface and reduce the adsorbed molecules; however, yields of MV+ as high as 6.5 radicals per 100 eV were measured, significantly exceeding common yields for ionization of less than 4.5 per 100 eV. The high yields suggest that an additional source for reduction equivalents is generated in the silica and is scavenged by the adsorbed MV2+ acceptor. Monte Carlo track structure calculations point to radiation-induced excitons as the scavengeable reducing species. The yield of MV+ radicals decreases linearly with increasing particle size at a given surface coverage by MV2+. This decrease in yield was attributed to a competition between electron-hole recombination within the silica bulk and scavenging of electrons at the interface. The limiting size at which no reducing equivalents arrive at the aqueous side of the interface was estimated to be dsimilar to30nm.