A matrix EPR spectroscopy study of the low temperature gamma radiolysis of precipitated (Zeosil) and mesoporous high surface silica has afforded evidence of the formation of trapped H-atoms, H-atom centers, siloxy radicals Si-O-center dot, anomalous silyl peroxy radicals Si-OO center dot with reduced g tensor anisotropy, siloxy radical-cations ( Si-O-Si )(+center dot), E' centers, and two species from Ge impurity. Coordination of peroxyl radicals with diamagnetic Si+ centers is proposed and tested by DFT computations in order to justify the observed g tensor. Coordination of H-atoms to Si+ centers is also proposed for the structure of the H-atom centers as an alternative model not requiring the intervention of Ge, Sn, or CO impurities. The DFT method has been employed to assess the electronic structure of siloxy radical-cations and its similarity with that of the carbon radical-cation analogues; the results have prompted a revision of the structures proposed in the literature for ST1 and ST2 centers. The comparison between the two types of silica has afforded evidence of different radiolysis mechanisms leading to a greater yield of trapped H-atoms and H-atom centers in zeosil silica, which is reckoned with the 4-fold greater concentration of silanol groups. Parallel radiolysis experiments carried out by using both types of silica with polybutadiene oligomers as adsorbate have afforded evidence of free valence and energy migration phenomena leading to irreversible linking of polybutadiene chains onto silica. Reaction mechanisms are proposed based on the detection of SiO2-bonded free radicals whose structure has been defined by EPR.