The reactivity of excited benzophenone (Bp) and 4,4 ' -dimethylbenzophenone (DMBp) in the presence of sodium cholate (NACh) aggregates was studied by following the kinetics of the excited triplet states and the ketyl radicals of both ketones. At low NACh concentrations only primary aggregates are present in solution. The ketyl radicals were formed from the reaction of the triplet ketones bound to the primary sites. The decay of the ketyl radicals occurred primarily by the reaction of these radicals in water. Some long-lived triplets included in the primary aggregate were also observed. A different reactivity was observed at higher NACh concentrations where secondary aggregates are present. The binding process associated with the secondary binding sites is much faster than for the primary site. The hydrogen abstraction reaction in the secondary binding site is too slow to compete with the exit process, but self-quenching competes efficiently, leading to a shortening of the triplet lifetimes for both ketones. From the dynamic results it was concluded that only a small number of NACh molecules (8-13) are needed to define the primary and secondary binding sites.