A Markovian theory is developed to study the efficiency of diffusion-reaction processes involving geminate radical pairs moving on the surface of a catalyst (e.g., zeolite) support. Results obtained for the simplest (lattice) system are corroborated by Monte Carlo calculations, and these are extended to study the effect of system size. A kinetic model is developed to account for the results obtained and leads to the general conclusion that the photochemical generation of a triplet pair, followed by the subsequent diffusion and eventual recombination of the radicals R up arrow and R down arrow, lengthens the mean reaction time by a factor of similar to2, relative to the case in which R up arrow and R down arrow are assumed to be present at the very outset.