Aqueous suspensions of methyl viologen, MV2+, adsorbed onto silica nanoparticles were irradiated using pulse radiolysis, and the yield of the radical cation was determined. It is shown that the yield of MV+ increases upon increasing the weight percent of SiO2 in the sample. This increase parallels the increase in the yield of hydrated electrons that was previously observed to result from an increase in the absorbed dose by the silica particles. The increase in adsorbate abundance at the surface changes the surface-charge density, but it is shown that the increase in the yield is not merely due to changes in the surface potential; an acceptor state at the surface is required. However, the possibility that the presence of an acceptor at the interface affects charge separation within the particle cannot be dismissed.