The swelling of microcrystalline cellulose by the use of polar protic solvents such as ethanol or methanol enables the penetration of benzophenone into submicroscopic pores of the natural polymer, while solvents such as benzene or dichloromethane do not open the polymer chains, thus not producing any entrapped benzophenone. Ground-state diffuse reflectance studies revealed a dramatic blue shift, in the 350-nm absorption of benzophenone in the former case, in accordance with a strong interaction of the hydroxyl groups of cellulose with the ketone. Diffuse reflectance laser flash photolysis studies of benzophenone adsorbed on microcrystalline cellulose showed, in cases where benzophenone is entrapped in the polymer chain, the formation of a transient which decays nonexponentially and exhibits a maximum absorption at about 530 nm, assigned to triplet benzophenone. After ca. 25 mu s, this transient generates another species with an absorption maximum at 545 nm. We assigned this new species to the diphenylketyl radical. In all cases where the solvent does not swell-cellulose, a different behavior was observed typical for benzophenone microcrystals triplet decay. The ketyl radical formation is greatly reduced in this case. Triplet benzophenone decays by complex kinetics and lives about 10 mu s when adsorbed onto microcrystalline cellulose, while the ketyl radical, when formed, lives 1 order of magnitude longer than the triplet. Samples which exhibit a high yield of ketyl radical formation also have a smaller phosphorescence emission in accordance with the fact that large amount of triplet molecules were consumed in the process of hydrogen abstraction from the matrix, involving hydrogens linked to carbons bearing a hydroxyl group.