When silica/polybutadiene and silica/styrene-butadiene are submitted to gamma irradiation at 77 K a scavenging of the silica radiolytic species by absorbed vinyl units takes place at the surface with concomitant grafting and crosslinking of the polymers. Key intermediate in such reactions are SiO2-bonded radicals of structure SiO2-C-center dot(CH3)CH(R)(R'). The role of silica is rationalized in terms of the following steps: a) absorption of the radiation energy in the bulk of the silica particles followed by exciton migration at the surface; b) reactions of excited groups (silanols) with absorbed vinyl units giving SiO2-bonded radicals; c) crosslinking of the polymers initiated by grafted radicals. (C) 2012 Elsevier Ltd. All rights reserved.