The production of H-2 in the radiolysis of silica suspensions in aqueous solutions and slurries has been examined. Radiation chemical yields of H-2 increase with increasing oxide weight fraction in suspensions Of SiO2 nanoparticles (7- and 22-nm diameter) probably because of the escape of electrons initially produced in the oxide to the bulk water. Reactions of H-2 with holes on the oxide surface or scavenging of precursors to H-2 in the bulk water decrease its yield at high SiO2 loading. The production of H-2 in the radiolysis of slurries of large SiO2 particles (8.1 m(2)/g, similar to343 nm diameter) is virtually identical to that expected in water alone suggesting little escape of electrons from the bulk oxide to the water. However, aqueous slurries of smaller SiO2 particles (312 m(2)/g, similar to8 nm diameter) can double the H-2 production from water. Studies with varying the concentration of selenate, a good scavenger of the hydrated electron and its precursor, suggest that electron reactions on the,us time domain are responsible for the additional H-2 produced in aqueous SiO2 mixtures. The only obvious difference between the 8-nm-particle slurry and the comparably sized suspension is the addition of additives in the latter for stability.