Dispersed silica colloids are used as model surfaces for studying triplet-state reaction kinetics at liquid/solid inter-faces. Phosphorescence from the triplet state of benzophenone is quenched by a hydrogen-atom abstraction from methyl groups immobilized on the surface of a silica colloid. The production of diphenylketyl radicals from this Norrish Type II reaction and their decay by recombination are also monitored using a time-resolved laser-induced fluorescence technique. Aggregation of the colloids results in a dispersion of benzophenone-triplet quenching rates; the fractions of excited-tripler states quenched by small clusters and larger aggregates were estimated and used to determine rate constants for reaction with the two populations of surface methyl groups. The rates of the interfacial reactions of triplet benzophenone were enhanced compared to reactions in free solution, which is likely due to weak adsorption of triplet benzophenone to the methylated silica surface.