A study of the quenching by molecular oxygen of the phosphorescence of benzophenone and camphorquinone adsorbed on silica gel under conditions of variable humidity and temperature is reported. A survey of the relevant models related to the adsorption and quenching reactions at the gas-solid interface is also presented. The triplet-state decay kinetics demonstrate an inverse power relationship which is best fitted by a semiempirical fractal model, suggesting that the reaction is affected by the geometrical features of the solid. The steady-state oxygen-quenching behavior of the adsorbed ketones exhibits some characteristics of a surface-mediated process whereby an oxygen migrates across several adsorption sites until it encounters an excited ketone; however, it is also likely that a major component of ballistic quenching occurs under these conditions. The influence of humidity may be readily explained if it is assumed that physisorbed water, present on the silica surface, inhibits both the surface diffusion of oxygen and the bombardment of the exposed probe by gas-phase oxygen.