Spectroscopic techniques are used to investigate energy transfer and radiation-induced reactions on the surfaces of particulate silicas, Upon subjection to high energy radiation, hydrogen atoms are produced with significant yields in all silica samples pretreated at 150 degrees C, for an example, G(H) = 4.2 in silica gel Davisil 60. Subsequent hydrogen addition to aromatic adsorbates such as pyrene, N,N-dimethylaniline, and methyl viologen (MV2+) proceeds on the surface in competition with the hydrogen dimerization, leading to the formation of their ii adducts. Hydrogen scavenging studies revealed that such addition reactions are heterogeneous in porous silica gel with contributions from a fast intrapore reaction within several nanoseconds and a much slower diffusion influenced interpore process over many microseconds. The efficient capture of H atoms by pyrene at a small percentage of surface coverage suggests that hydrogen is produced at the specific sites where pyrene adsorbs. Electron scavenging by CHCl3, Cd2+, and MV2+ leads to a suppression of II addition and an enhanced positive charge transfer to pyrene. Pretreatment of silica samples al 600 degrees C gives rise to increased ionic products of aromatic adsorbates, concomitant with decreased hydrogen production. These experimental observations are understood in terms of an exciton model. By comparison with the electronic processes in bulk. silica materials, it is shown that surface plays a major role in the relaxation of excitation and the activation of adsorbate chemistry.