Benzophenone and fluorenone, which have a nonrigid and a rigid structure, respectively, were used as probes to study the nature of the adsorption process onto microcrystalline cellulose. Diffuse reflectance techniques were used in the UV-vis and infrared regions. Luminescence studies revealed that whenever fluorenone or benzophenone are entrapped into the natural polymer chains and in close contact with the substrate, a strong quenching effect exists for both probe’s luminescence at room temperature. For fluorenone, the fluorescence quantum yields (Phi(F)) determined were about 0.10 when dichloromethane, cyclohexane, and benzene (solvents which do not swell cellulose) were used for sample preparation, while for dioxane, acetone, ethanol, and methanol(solvents which efficiently swell cellulose) Phi(F) was approximately 0.01. These values are about 1 order of magnitude higher than those obtained in solution, showing the importance of the rigid dry matrix in reducing the nonradiative pathways of deactivation of the (pi,pi*) fluorenone first excited singlet state. Complementary, infrared studies showed that the carbonyl group of benzophenone is affected by entrapment (when the solvents used induce the swelling of cellulose), whereas in fluorenone the same band is insensitive to the adsorption process, not allowing the differentiation between entrapped molecules and surface crystallites of this ketone. These observations implied that benzophenone is entrapped between the chains of the polymer forming hydrogen bonds between the carbonyl and the hydroxyl groups of the glycosidic chains, while the rigidity of fluorenone apparently restrains the ketone-substrate interactions to the aromatic rings. Through the modifications observed in the carbonyl stretching band of benzophenone, it was possible to establish a swelling effect scale for the solvents, which is compared with previous results.